diff --git a/src/zm_image.cpp b/src/zm_image.cpp
index ccf37e0e0..e10ebb4b6 100644
--- a/src/zm_image.cpp
+++ b/src/zm_image.cpp
@@ -1248,6 +1248,33 @@ Image *Image::HighlightEdges(
         }
       }
     }
+  } else if ( zm_is_yuv420(p_pixfmt) ) {
+    // Single alarm colour over a transparent (Y=0) background; Overlay() onto
+    // a YUV420 image keys on a non-zero luma marker. Write the colour's luma
+    // at each edge pixel and its chroma at the shared 2x2 chroma sample.
+    const YUV yuv = brg_to_yuv(colour);
+    const uint8_t Yc = Y_VAL(yuv), Uc = U_VAL(yuv), Vc = V_VAL(yuv);
+    uint8_t *hplane[4] = {};
+    int hstride[4] = {};
+    if (av_image_fill_arrays(hplane, hstride, high_buff, p_pixfmt, width, height, 32) < 0) {
+      Error("HighlightEdges: av_image_fill_arrays failed for YUV420 %ux%u", width, height);
+      return high_image;
+    }
+    for ( unsigned int y = lo_y; y <= hi_y; y++ ) {
+      const uint8_t* p = buffer + (y * src_linesize) + lo_x;
+      for ( unsigned int x = lo_x; x <= hi_x; x++, p++ ) {
+        bool edge = false;
+        if ( *p ) {
+          edge = (x > 0 && !*(p-1)) || (x < (width-1) && !*(p+1))
+              || (y > 0 && !*(p-src_linesize)) || (y < (height-1) && !*(p+src_linesize));
+        }
+        if ( edge ) {
+          hplane[0][y * hstride[0] + x] = Yc ? Yc : 1;  // keep the luma marker non-zero
+          hplane[1][(y / 2) * hstride[1] + (x / 2)] = Uc;
+          hplane[2][(y / 2) * hstride[2] + (x / 2)] = Vc;
+        }
+      }
+    }
   }
 
   return high_image;
@@ -1998,8 +2025,51 @@ void Image::Overlay( const Image &image ) {
   // chroma. After Colourise() the destination's linesize is updated to the
   // new format's stride, so we re-read it inside each branch.
 
-  /* Grayscale/YUV420 on top of grayscale/YUV420 - complete */
-  if ( zm_bytes_per_pixel(imagePixFormat) == 1 && zm_bytes_per_pixel(image.imagePixFormat) == 1 ) {
+  /* YUV420 on top of YUV420 - copy luma + chroma using luma as the mask */
+  if ( zm_is_yuv420(imagePixFormat) && zm_is_yuv420(image.imagePixFormat) ) {
+    // The overlay (a zone alarm highlight) is built in the target's format
+    // with a Clear()ed (Y=0) transparent background, so a non-zero source
+    // luma marks a pixel to paint. Copy that luma, and copy the shared
+    // chroma sample whenever any of the luma pixels it covers is marked.
+    uint8_t *dplane[4] = {};
+    int dstride[4] = {};
+    const uint8_t *splane[4] = {};
+    int sstride[4] = {};
+    if (av_image_fill_arrays(dplane, dstride, buffer, imagePixFormat, width, height, 32) < 0
+        || av_image_fill_arrays(const_cast<uint8_t **>(splane), sstride, image.buffer,
+                                image.imagePixFormat, width, height, 32) < 0) {
+      Error("Overlay: av_image_fill_arrays failed for YUV420 %ux%u", width, height);
+      return;
+    }
+    for (unsigned int y = 0; y < height; y++) {
+      const uint8_t *psrc = splane[0] + y * sstride[0];
+      uint8_t *pdest = dplane[0] + y * dstride[0];
+      for (unsigned int x = 0; x < width; x++) {
+        if (psrc[x]) pdest[x] = psrc[x];
+      }
+    }
+    const unsigned int cw = (width + 1) / 2;
+    const unsigned int ch = (height + 1) / 2;
+    for (unsigned int cy = 0; cy < ch; cy++) {
+      for (unsigned int cx = 0; cx < cw; cx++) {
+        bool marked = false;
+        for (unsigned int dy = 0; dy < 2 && !marked; dy++) {
+          const unsigned int ly = cy * 2 + dy;
+          if (ly >= height) break;
+          for (unsigned int dx = 0; dx < 2; dx++) {
+            const unsigned int lx = cx * 2 + dx;
+            if (lx < width && splane[0][ly * sstride[0] + lx]) { marked = true; break; }
+          }
+        }
+        if (marked) {
+          dplane[1][cy * dstride[1] + cx] = splane[1][cy * sstride[1] + cx];
+          dplane[2][cy * dstride[2] + cx] = splane[2][cy * sstride[2] + cx];
+        }
+      }
+    }
+
+    /* Grayscale/YUV420 on top of grayscale/YUV420 - complete */
+  } else if ( zm_bytes_per_pixel(imagePixFormat) == 1 && zm_bytes_per_pixel(image.imagePixFormat) == 1 ) {
     // Overlay only the luma/primary plane. Width is shared (panic above).
     for (unsigned int y = 0; y < height; y++) {
       const uint8_t *psrc = image.buffer + y * image.linesize;
diff --git a/src/zm_zone.cpp b/src/zm_zone.cpp
index ca810b75e..edace6d09 100644
--- a/src/zm_zone.cpp
+++ b/src/zm_zone.cpp
@@ -790,7 +790,13 @@ bool Zone::CheckAlarms(const Image *delta_image) {
         }
       }  // end for y
 
-      if (monitor->Colours() == ZM_COLOUR_GRAY8) {
+      // Build the alarm highlight in the capture's own pixel format so Overlay()
+      // onto the analysis image is a same-format copy. monitor->Colours()==1
+      // aliases both GRAY8 and planar YUV420P, so resolve the real format first:
+      // a true GRAY8 monitor has no colour to carry, so upgrade its highlight to
+      // RGB24; YUV420P keeps its format and carries the alarm colour in chroma.
+      AVPixelFormat capture_fmt = zm_pixformat_from_colours(monitor->Colours(), monitor->SubpixelOrder());
+      if (capture_fmt == AV_PIX_FMT_GRAY8) {
         image = diff_image->HighlightEdges(alarm_rgb, ZM_COLOUR_RGB24, ZM_SUBPIX_ORDER_RGB, &polygon.Extent());
       } else {
         image = diff_image->HighlightEdges(alarm_rgb, monitor->Colours(), monitor->SubpixelOrder(), &polygon.Extent());
diff --git a/tests/zm_image.cpp b/tests/zm_image.cpp
index 4405c5e49..0585eaf2a 100644
--- a/tests/zm_image.cpp
+++ b/tests/zm_image.cpp
@@ -207,3 +207,79 @@ TEST_CASE("Image::Flip YUV420P", "[image]") {
     REQUIRE(dst.data[1][(h / 2 - 1 - 30) * dst.stride[1] + 40] == 210);
   }
 }
+
+// Regression: a zone alarm highlight (RGB) overlaid onto a planar YUV420P
+// analysis image used to call Colourise() — which only handles GRAY8, so it
+// bailed with "already colourised, colours: 1" (the GRAY8/YUV420P alias) and
+// then walked the 1.5x planar buffer as 3x packed RGB (OOB write). Highlights
+// are now built in the target's YUV420P format and Overlay() copies luma plus
+// the shared chroma sample, keyed on a non-zero source luma marker.
+TEST_CASE("Image::Overlay YUV420P highlight", "[image]") {
+  bootstrap_image_config();
+  const int w = 64, h = 48;
+
+  Image target(w, h, ZM_COLOUR_GRAY8, ZM_SUBPIX_ORDER_YUV420P);
+  Planes tgt = plane_view(target, AV_PIX_FMT_YUV420P, w, h);
+  memset(tgt.data[0], 16, static_cast<size_t>(tgt.stride[0]) * h);     // dim luma
+  memset(tgt.data[1], 128, static_cast<size_t>(tgt.stride[1]) * (h / 2));  // neutral chroma
+  memset(tgt.data[2], 128, static_cast<size_t>(tgt.stride[2]) * (h / 2));
+
+  // Transparent (Y=0) highlight with a 2x2 luma marker fully covering chroma
+  // sample (5,5), plus a single-pixel marker partially covering sample (10,10).
+  Image high(w, h, ZM_COLOUR_GRAY8, ZM_SUBPIX_ORDER_YUV420P);
+  Planes hi = plane_view(high, AV_PIX_FMT_YUV420P, w, h);
+  memset(hi.data[0], 0, static_cast<size_t>(hi.stride[0]) * h);
+  memset(hi.data[1], 0, static_cast<size_t>(hi.stride[1]) * (h / 2));
+  memset(hi.data[2], 0, static_cast<size_t>(hi.stride[2]) * (h / 2));
+  for (int y : {10, 11}) for (int x : {10, 11}) hi.data[0][y * hi.stride[0] + x] = 200;
+  hi.data[1][5 * hi.stride[1] + 5] = 84;   // red-ish chroma
+  hi.data[2][5 * hi.stride[2] + 5] = 255;
+  hi.data[0][20 * hi.stride[0] + 20] = 150;  // single covering pixel of sample (10,10)
+  hi.data[1][10 * hi.stride[1] + 10] = 43;   // green-ish chroma
+  hi.data[2][10 * hi.stride[2] + 10] = 21;
+
+  target.Overlay(high);
+
+  Planes out = plane_view(target, AV_PIX_FMT_YUV420P, w, h);
+  // Marked luma copied; unmarked luma untouched.
+  for (int y : {10, 11}) for (int x : {10, 11})
+    CHECK(out.data[0][y * out.stride[0] + x] == 200);
+  CHECK(out.data[0][20 * out.stride[0] + 20] == 150);
+  CHECK(out.data[0][0] == 16);
+  CHECK(out.data[0][5 * out.stride[0] + 5] == 16);
+  // Chroma copied for any covered sample; untouched elsewhere.
+  CHECK(out.data[1][5 * out.stride[1] + 5] == 84);
+  CHECK(out.data[2][5 * out.stride[2] + 5] == 255);
+  CHECK(out.data[1][10 * out.stride[1] + 10] == 43);
+  CHECK(out.data[2][10 * out.stride[2] + 10] == 21);
+  CHECK(out.data[1][0] == 128);
+  CHECK(out.data[2][0] == 128);
+}
+
+// HighlightEdges must be able to emit a YUV420P highlight (so it can be
+// overlaid onto a YUV420P analysis image in the same format). A filled blob's
+// border pixels become non-zero luma markers carrying the alarm chroma.
+TEST_CASE("Image::HighlightEdges YUV420P output", "[image]") {
+  bootstrap_image_config();
+  const int w = 64, h = 48;
+
+  Image src(w, h, ZM_COLOUR_GRAY8, ZM_SUBPIX_ORDER_NONE);
+  src.Clear();
+  // Fill an interior rectangle so it has edges away from the image border.
+  for (int y = 10; y < 30; y++)
+    for (int x = 10; x < 40; x++)
+      src.Buffer()[y * src.LineSize() + x] = 255;
+
+  Rgb red = kRGBRed;  // alarm colour
+  Image *high = src.HighlightEdges(red, ZM_COLOUR_GRAY8, ZM_SUBPIX_ORDER_YUV420P, nullptr);
+  REQUIRE(high != nullptr);
+  REQUIRE(high->PixFormat() == AV_PIX_FMT_YUV420P);
+
+  Planes hi = plane_view(*high, AV_PIX_FMT_YUV420P, w, h);
+  // A border pixel of the blob must carry a non-zero luma marker; an interior
+  // pixel (not an edge) and an outside pixel must stay transparent (Y=0).
+  CHECK(hi.data[0][10 * hi.stride[0] + 20] != 0);  // top edge
+  CHECK(hi.data[0][20 * hi.stride[0] + 25] == 0);  // interior, not an edge
+  CHECK(hi.data[0][0] == 0);                        // outside the blob
+  delete high;
+}