Fix PixelColorType.YCbCr wrongly interpreted as grayscale in PngEncoder

src/ImageSharp/Formats/Png/PngMetadata.cs

@@ -111,7 +111,7 @@ public class PngMetadata : IFormatMetadata<PngMetadata>
                 color = PngColorType.Rgb;
                 break;
             default:
-                if (colorType.HasFlag(PixelColorType.Luminance))
+                if (colorType.HasFlag(PixelColorType.Luminance | PixelColorType.Alpha))
                 {
                     color = PngColorType.GrayscaleWithAlpha;
                     break;

tests/ImageSharp.Tests/Formats/Png/PngMetadataTests.cs

@@ -336,4 +336,29 @@ public class PngMetadataTests
 
         Assert.Equal(42, (int)exif.GetValue(ExifTag.ImageNumber).Value);
     }
+
+
+    [Theory]
+    [InlineData(PixelColorType.Binary, PngColorType.Palette)]
+    [InlineData(PixelColorType.Indexed, PngColorType.Palette)]
+    [InlineData(PixelColorType.Luminance, PngColorType.Grayscale)]
+    [InlineData(PixelColorType.RGB, PngColorType.Rgb)]
+    [InlineData(PixelColorType.BGR, PngColorType.Rgb)]
+    [InlineData(PixelColorType.YCbCr, PngColorType.RgbWithAlpha)]
+    [InlineData(PixelColorType.CMYK, PngColorType.RgbWithAlpha)]
+    [InlineData(PixelColorType.YCCK, PngColorType.RgbWithAlpha)]
+    public void FromFormatConnectingMetadata_ConvertColorTypeAsExpected(PixelColorType pixelColorType, PngColorType expectedPngColorType)
+    {
+        FormatConnectingMetadata formatConnectingMetadata = new()
+        {
+            PixelTypeInfo = new PixelTypeInfo(24)
+            {
+                ColorType = pixelColorType,
+            },
+        };
+
+        PngMetadata actual = PngMetadata.FromFormatConnectingMetadata(formatConnectingMetadata);
+
+        Assert.Equal(expectedPngColorType, actual.ColorType);
+    }
 }