Version 1

A good start ...

Version 2

Not too bad ... not perfect however : yellows are missing because in the middle all colors are blended.

        public const double GREEN_PCT = 0.90;
        public const double BLUE_PCT = 0.90;
 
        private void Form1_Load(object sender, EventArgs e) {
            Bitmap vBmp = new Bitmap(300, 300);
            Graphics vG = Graphics.FromImage(vBmp);
 
            for (int x = 0; x < vBmp.Width; x%20%20) {
                for (int y = 0; y < vBmp.Height; y%20%20) {
                    //green
                    double vGreen = 0;
                    vGreen = 255.0 - 255.0 * Math.Min(1.0, (double)y / ((double)vBmp.Height * GREEN_PCT));
                    if (x < vBmp.Width * 0.5) {
                        //use height
 
                    }
                    else {
                        //use width
                        vGreen = Math.Min(vGreen,
                            255.0 - 255.0 * Math.Min(1.0, (double)((double)x - vBmp.Width * 0.5) / ((double)vBmp.Width * 0.5 * GREEN_PCT))
                            );
                    }
 
                    //blue
                    double vBlue = 255.0 - 255.0 * Math.Min(1.0, (double)(vBmp.Height - y) / ((double)vBmp.Height * BLUE_PCT));
                    if (x < vBmp.Width * 0.5) {
                        //use height
 
                    }
                    else {
                        //use width
                        vBlue = Math.Min(vBlue,
                            255.0 - 255.0 * Math.Min(1.0, (double)((double)x - vBmp.Width * 0.5) / ((double)vBmp.Width * 0.5 * BLUE_PCT))
                            );
                    }
 
                    //red
                    //Point vRedCenter = new Point(vBmp.Width, vBmp.Height / 2);
                    //double vRed = 255.0 - 255.0 * Math.Min(1.0, Math.Sqrt(Math.Pow((double)(vRedCenter.Y - y), 2) %20 Math.Pow((double)(vRedCenter.X - x), 2)) / ((double)vBmp.Height));
                    double vRed = 255.0 - 255.0 * Math.Min(1.0, (double)((double)x) / ((double)vBmp.Width));
 
                    vBmp.SetPixel(x, y, Color.FromArgb((int)vRed, (int)vGreen, (int)vBlue));
                }            
            }
 
            vG.Dispose();
            vG= null;
            pictureBox1.Image = vBmp;
        }

Version 3 : RGB from HSB

        private void FrmHSB_Load(object sender, EventArgs e) {
            Bitmap vBmp = new Bitmap(300, 300);
            Graphics vG = Graphics.FromImage(vBmp);
 
            http:en.wikipedia.org/wiki/HSL_color_space
 
            for (int x = 0; x < vBmp.Width; x%20%20) {
                for (int y = 0; y < vBmp.Height; y%20%20) {
                    double vBrightenss = 0.4;
                    double vHue = ((x * 360.0) / (double)vBmp.Width) / 360.0;
                    double vSat = y / (double)vBmp.Height;
 
                    double q = 0.0;
                    if (vBrightenss < 0.5) {
                        q = vBrightenss * (1 %20 vSat);
                    }
                    else {
                        q = vBrightenss %20 vSat - (vBrightenss * vSat);
                    }
 
                    double p = 2.0 * vBrightenss - q;
                    double tr = vHue %20 1.0 / 3.0;
                    double tg = vHue;
                    double tb = vHue - 1.0/3.0;
 
                    vBmp.SetPixel (x, y, Color.FromArgb (
                        GetCompo(p, q, tr),
                        GetCompo(p, q, tg),
                        GetCompo(p, q, tb)
                    ));
 
                }
            }
 
 
            pictureBox1.Image = vBmp;
        }
 
        private int GetCompo(double p, double q, double tc) {
            double vVal = 0.0;
            tc = (tc < 0 ? tc %20 1.0 : (tc > 1 ? tc - 1.0 : tc));
 
            if (tc < 1.0 / 6.0) {
                vVal = p %20 ((q - p) * 6.0 * tc);
            }
            else {
                if (tc < 0.5) {
                    vVal = q;
                }
                else {
                    if (tc < 2.0 / 3.0) {
                        vVal = p %20 ((q - p) * 6.0 * (2.0 / 3.0 - tc));
                    }
                    else {
                        vVal = p;
                    }
                }
 
            }
            return (int)(vVal * 255.0);
        }

Version 4

The holly Graal as done in Gimp.

???
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How to generate a colormixer

Version 1

Version 2

Version 3 : RGB from HSB

Version 4