diff --git a/docs/changelog.md b/docs/changelog.md
index 174937ca..cf448cae 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -4,6 +4,11 @@
 
 Requires libvips v8.11.3
 
+### v0.29.1 - TBD
+
+* Ensure correct PNG bitdepth is set based on number of colours.
+  [#2855](https://github.com/lovell/sharp/issues/2855)
+
 ### v0.29.0 - 17th August 2021
 
 * Drop support for Node.js 10, now requires Node.js >= 12.13.0.
diff --git a/lib/constructor.js b/lib/constructor.js
index 02872bd0..c76f365f 100644
--- a/lib/constructor.js
+++ b/lib/constructor.js
@@ -232,7 +232,7 @@ const Sharp = function (input, options) {
     pngAdaptiveFiltering: false,
     pngPalette: false,
     pngQuality: 100,
-    pngColours: 256,
+    pngBitdepth: 8,
     pngDither: 1,
     webpQuality: 80,
     webpAlphaQuality: 100,
diff --git a/lib/output.js b/lib/output.js
index 6ec1b125..e0b2610c 100644
--- a/lib/output.js
+++ b/lib/output.js
@@ -405,7 +405,7 @@ function png (options) {
       const colours = options.colours || options.colors;
       if (is.defined(colours)) {
         if (is.integer(colours) && is.inRange(colours, 2, 256)) {
-          this.options.pngColours = colours;
+          this.options.pngBitdepth = 1 << 31 - Math.clz32(Math.ceil(Math.log2(colours)));
         } else {
           throw is.invalidParameterError('colours', 'integer between 2 and 256', colours);
         }
diff --git a/src/pipeline.cc b/src/pipeline.cc
index 35eff31b..e11de97f 100644
--- a/src/pipeline.cc
+++ b/src/pipeline.cc
@@ -799,7 +799,7 @@ class PipelineWorker : public Napi::AsyncWorker {
             ->set("filter", baton->pngAdaptiveFiltering ? VIPS_FOREIGN_PNG_FILTER_ALL : VIPS_FOREIGN_PNG_FILTER_NONE)
             ->set("palette", baton->pngPalette)
             ->set("Q", baton->pngQuality)
-            ->set("colours", baton->pngColours)
+            ->set("bitdepth", baton->pngBitdepth)
             ->set("dither", baton->pngDither)));
           baton->bufferOut = static_cast<char*>(area->data);
           baton->bufferOutLength = area->length;
@@ -955,7 +955,7 @@ class PipelineWorker : public Napi::AsyncWorker {
             ->set("filter", baton->pngAdaptiveFiltering ? VIPS_FOREIGN_PNG_FILTER_ALL : VIPS_FOREIGN_PNG_FILTER_NONE)
             ->set("palette", baton->pngPalette)
             ->set("Q", baton->pngQuality)
-            ->set("colours", baton->pngColours)
+            ->set("bitdepth", baton->pngBitdepth)
             ->set("dither", baton->pngDither));
           baton->formatOut = "png";
         } else if (baton->formatOut == "webp" || (mightMatchInput && isWebp) ||
@@ -1429,7 +1429,7 @@ Napi::Value pipeline(const Napi::CallbackInfo& info) {
   baton->pngAdaptiveFiltering = sharp::AttrAsBool(options, "pngAdaptiveFiltering");
   baton->pngPalette = sharp::AttrAsBool(options, "pngPalette");
   baton->pngQuality = sharp::AttrAsUint32(options, "pngQuality");
-  baton->pngColours = sharp::AttrAsUint32(options, "pngColours");
+  baton->pngBitdepth = sharp::AttrAsUint32(options, "pngBitdepth");
   baton->pngDither = sharp::AttrAsDouble(options, "pngDither");
   baton->webpQuality = sharp::AttrAsUint32(options, "webpQuality");
   baton->webpAlphaQuality = sharp::AttrAsUint32(options, "webpAlphaQuality");
diff --git a/src/pipeline.h b/src/pipeline.h
index d934ded2..3beb9713 100644
--- a/src/pipeline.h
+++ b/src/pipeline.h
@@ -146,7 +146,7 @@ struct PipelineBaton {
   bool pngAdaptiveFiltering;
   bool pngPalette;
   int pngQuality;
-  int pngColours;
+  int pngBitdepth;
   double pngDither;
   int webpQuality;
   int webpAlphaQuality;
@@ -276,7 +276,7 @@ struct PipelineBaton {
     pngAdaptiveFiltering(false),
     pngPalette(false),
     pngQuality(100),
-    pngColours(256),
+    pngBitdepth(8),
     pngDither(1.0),
     webpQuality(80),
     webpAlphaQuality(100),