C# code to map the 24-bit colours in an image to the nearest 8 or 9-bit colours.

ColourMapper.cs

using System;
using System.Drawing;
using System.Drawing.Imaging;
using JetBrains.Annotations;

namespace Elena.Compiler.Console
{
    /// <summary>
    /// Maps the colours in a bitmap to the nearest Next colour. Uses the palettes from <see cref="PaletteGenerator" />.
    /// (https://gist.github.com/MrKWatkins/ea62814ed78b79edf8c5aa4902652ff3)
    /// </summary>
    public static class ColourMapper
    {
        public static void MapTo8Bit([NotNull] [PathReference] string input, [NotNull] [PathReference] string output) => MapTo8Bit(input, output, ImageFormat.Png);

        public static void MapTo8Bit([NotNull] [PathReference] string input, [NotNull] [PathReference] string output, [NotNull] ImageFormat outputFormat)
        {
            Map(input, output, outputFormat, MapTo2Bits);
        }

        public static void MapTo9Bit([NotNull] [PathReference] string input, [NotNull] [PathReference] string output) => MapTo9Bit(input, output, ImageFormat.Png);

        public static void MapTo9Bit([NotNull] [PathReference] string input, [NotNull] [PathReference] string output, [NotNull] ImageFormat outputFormat)
        {
            Map(input, output, outputFormat, MapTo3Bits);
        }

        private static void Map([NotNull] [PathReference] string input, [NotNull] [PathReference] string output, [NotNull] ImageFormat outputFormat, Func<byte, byte> bMap)
        {
            using (var source = (Bitmap) Image.FromFile(input))
            using (var target = new Bitmap(source.Width, source.Height))
            {
                for (var x = 0; x < source.Width; x++)
                {
                    for (var y = 0; y < source.Height; y++)
                    {
                        var originalColour = source.GetPixel(x, y);
                        var newColour = Color.FromArgb(MapTo3Bits(originalColour.R), MapTo3Bits(originalColour.G), bMap(originalColour.B));
                        target.SetPixel(x, y, newColour);
                    }
                }

                target.Save(output, outputFormat);
            }
        }

        [Pure]
        private static byte MapTo3Bits(byte component) => (byte) ((component >> 5) * PaletteGenerator.Step3Bit);

        [Pure]
        private static byte MapTo2Bits(byte component) => (byte) ((component >> 5) * PaletteGenerator.Step2Bit);
    }
}