You have three colour channels 0 to 255 R, G and B.

First go through

0, 0, 255
0, 255, 0
255, 0, 0

Then go through

0, 255, 255
255, 0, 255
255, 255, 0

Then divide by 2 => 128 and start again:

0, 0, 128
0, 128, 0
128, 0, 0
0, 128, 128
128, 0, 128
128, 128, 0

Divide by 2 => 64

Next time add 64 to 128 => 192

follow the pattern.

Straightforward to program and gives you fairly distinct colours.

EDIT: Request for code sample

Also – adding in the additional pattern as below if gray is an acceptable colour:

255, 255, 255
128, 128, 128 

There are a number of ways you can handle generating these in code.

The Easy Way

If you can guarantee that you will never need more than a fixed number of colours, just generate an array of colours following this pattern and use those:

    static string[] ColourValues = new string[] { 
        "FF0000", "00FF00", "0000FF", "FFFF00", "FF00FF", "00FFFF", "000000", 
        "800000", "008000", "000080", "808000", "800080", "008080", "808080", 
        "C00000", "00C000", "0000C0", "C0C000", "C000C0", "00C0C0", "C0C0C0", 
        "400000", "004000", "000040", "404000", "400040", "004040", "404040", 
        "200000", "002000", "000020", "202000", "200020", "002020", "202020", 
        "600000", "006000", "000060", "606000", "600060", "006060", "606060", 
        "A00000", "00A000", "0000A0", "A0A000", "A000A0", "00A0A0", "A0A0A0", 
        "E00000", "00E000", "0000E0", "E0E000", "E000E0", "00E0E0", "E0E0E0", 
    };

The Hard Way

If you don’t know how many colours you are going to need, the code below will generate up to 896 colours using this pattern. (896 = 256 * 7 / 2) 256 is the colour space per channel, we have 7 patterns and we stop before we get to colours separated by only 1 colour value.

I’ve probably made harder work of this code than I needed to. First, there is an intensity generator which starts at 255, then generates the values as per the pattern described above. The pattern generator just loops through the seven colour patterns.

using System;

class Program {
    static void Main(string[] args) {
        ColourGenerator generator = new ColourGenerator();
        for (int i = 0; i < 896; i++) {
            Console.WriteLine(string.Format("{0}: {1}", i, generator.NextColour()));
        }
    }
}

public class ColourGenerator {

    private int index = 0;
    private IntensityGenerator intensityGenerator = new IntensityGenerator();

    public string NextColour() {
        string colour = string.Format(PatternGenerator.NextPattern(index),
            intensityGenerator.NextIntensity(index));
        index++;
        return colour;
    }
}

public class PatternGenerator {
    public static string NextPattern(int index) {
        switch (index % 7) {
        case 0: return "{0}0000";
        case 1: return "00{0}00";
        case 2: return "0000{0}";
        case 3: return "{0}{0}00";
        case 4: return "{0}00{0}";
        case 5: return "00{0}{0}";
        case 6: return "{0}{0}{0}";
        default: throw new Exception("Math error");
        }
    }
}

public class IntensityGenerator {
    private IntensityValueWalker walker;
    private int current;

    public string NextIntensity(int index) {
        if (index == 0) {
            current = 255;
        }
        else if (index % 7 == 0) {
            if (walker == null) {
                walker = new IntensityValueWalker();
            }
            else {
                walker.MoveNext();
            }
            current = walker.Current.Value;
        }
        string currentText = current.ToString("X");
        if (currentText.Length == 1) currentText = "0" + currentText;
        return currentText;
    }
}

public class IntensityValue {

    private IntensityValue mChildA;
    private IntensityValue mChildB;

    public IntensityValue(IntensityValue parent, int value, int level) {
        if (level > 7) throw new Exception("There are no more colours left");
        Value = value;
        Parent = parent;
        Level = level;
    }

    public int Level { get; set; }
    public int Value { get; set; }
    public IntensityValue Parent { get; set; }

    public IntensityValue ChildA {
        get {
            return mChildA ?? (mChildA = new IntensityValue(this, this.Value - (1<<(7-Level)), Level+1));
        }
    }

    public IntensityValue ChildB {
        get {
            return mChildB ?? (mChildB = new IntensityValue(this, Value + (1<<(7-Level)), Level+1));
        }
    }
}

public class IntensityValueWalker {

    public IntensityValueWalker() {
        Current = new IntensityValue(null, 1<<7, 1);
    }

    public IntensityValue Current { get; set; }

    public void MoveNext() {
        if (Current.Parent == null) {
            Current = Current.ChildA;
        }
        else if (Current.Parent.ChildA == Current) {
            Current = Current.Parent.ChildB;
        }
        else {
            int levelsUp = 1;
            Current = Current.Parent;
            while (Current.Parent != null && Current == Current.Parent.ChildB) {
                Current = Current.Parent;
                levelsUp++;
            }
            if (Current.Parent != null) {
                Current = Current.Parent.ChildB;
            }
            else {
                levelsUp++;
            }
            for (int i = 0; i < levelsUp; i++) {
                Current = Current.ChildA;
            }

        }
    }
}