Use this.maxColors when getting size of the reduced palette, fixes #1505

src/ImageSharp/Processing/Processors/Quantization/OctreeQuantizer{TPixel}.cs

@@ -6,7 +6,6 @@ using System.Buffers;
 using System.Numerics;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
-using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.PixelFormats;
 
@@ -96,7 +95,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
             this.octree.Palletize(paletteSpan, this.maxColors, ref paletteIndex);
 
             // Length of reduced palette + transparency.
-            ReadOnlyMemory<TPixel> result = this.paletteOwner.Memory.Slice(0, Math.Min(paletteIndex + 2, QuantizerConstants.MaxColors));
+            ReadOnlyMemory<TPixel> result = this.paletteOwner.Memory.Slice(0, Math.Min(paletteIndex + 2, this.maxColors));
             this.pixelMap = new EuclideanPixelMap<TPixel>(this.Configuration, result);
 
             this.palette = result;