Snippet from a WPF app that produces an image where every pixel is a different color.

MultiColorImageCreator

using System.Drawing;
using System.Drawing.Imaging;
using System.Windows;

namespace GenerateMultiColorImages
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            var imgHeight = 128;
            var imgWidth = 768;

            var bmp = new Bitmap(imgWidth, imgHeight);

            var totalColors = 256*256*256;
            var colorRequired = imgHeight * imgWidth;

            var r = 0;
            var g = 0;
            var b = 0;

            var skip = int.Parse((totalColors/colorRequired).ToString()) - 1;

            for (var x = 0; x < imgWidth; x++)
            {
                for (var y = 0; y < imgHeight; y++)
                {
                    bmp.SetPixel(x, y, Color.FromArgb(r, g, b));

                    b += skip;

                    if (b > 255)
                    {
                        b = b - 255;
                        g += 1;
                    }

                    if (g > 255)
                    {
                        g = g - 255;
                        r += 1;
                    }
                }
            }

            bmp.Save("generated.png", ImageFormat.Png);
        }
    }
}