C# code to generate GIMP .gpl palette files for 8 and 9-bit ZX Spectrum Next palettes

PaletteGenerator.cs

using System.Collections.Generic;
using System.Drawing;
using System.IO;
using JetBrains.Annotations;

namespace Elena.Compiler.Console
{
    /// <summary>
    /// Utility class to write out 8 and 9-bit Next palettes in Gimp GPL format.
    /// </summary>
    public static class PaletteGenerator
    {
        // These steps are used to interpolate the full 255 range over the relevant number of bits. They split the range evenly
        // and the first 2/3 bits of the interpolated value are the 2/3 bit value, meaning we can just trim the first 2/3 bits
        // of the interpolated value to get the Next value.
        internal const byte Step2Bit = 85;
        internal const byte Step3Bit = 36;

        public static void Write8BitToFile([NotNull] [PathReference] string path)
        {
            using (var writer = File.CreateText(path))
            {
                WriteHeader(writer, "ZX Spectrum Next 8-Bit");

                WriteColours(writer, Enumerate8BitColours());
            }
        }

        public static void Write9BitToFile([NotNull] [PathReference] string path)
        {
            using (var writer = File.CreateText(path))
            {
                WriteHeader(writer, "ZX Spectrum Next 9-Bit");
                
                WriteColours(writer, Enumerate9BitColours());
            }
        }
        
        [Pure]
        [NotNull]
        [LinqTunnel]
        public static IEnumerable<Color> Enumerate8BitColours() => EnumerateColours(Step2Bit);
        
        [Pure]
        [NotNull]
        [LinqTunnel]
        public static IEnumerable<Color> Enumerate9BitColours() => EnumerateColours(Step3Bit);

        [Pure]
        [LinqTunnel]
        private static IEnumerable<Color> EnumerateColours(byte bStep)
        {
            for (var r = 0; r < 256; r+=Step3Bit)
            for (var g = 0; g < 256; g+=Step3Bit)
            for (var b = 0; b < 256; b+=bStep)
            {
                yield return Color.FromArgb(r, g, b);
            }
        }

        private static void WriteColours([NotNull] TextWriter writer, [NotNull] [InstantHandle] IEnumerable<Color> colours, int startIndex = 0)
        {
            var index = startIndex;
            foreach (var colour in colours)
            {
                WriteComponent(writer, colour.R);
                writer.Write(' ');
                WriteComponent(writer, colour.G);
                writer.Write(' ');
                WriteComponent(writer, colour.B);
                writer.Write(" Index ");
                writer.WriteLine(index);
                index++;
            }
        }

        private static void WriteComponent([NotNull] TextWriter writer, byte component)
        {
            writer.Write(component.ToString().PadLeft(3, ' '));
        }

        private static void WriteHeader([NotNull] TextWriter writer, [NotNull] string name)
        {
            writer.WriteLine("GIMP Palette");

            writer.Write("Name: ");
            writer.WriteLine(name);

            writer.WriteLine("Columns: 16");

            writer.WriteLine("#");
        }
    }
}