Rainbow Gradient

ColorUtil.cs

using System;
using System.Drawing;

public class ColorUtil
{
    public static Color GetRainbowColor(float percentage)
    {
    	// The code below tries to mimick Matlab's color gradient. A naive implementation which
    	// linearly interpolates hue, ends up with a very large range mapped to greenish colors
    	// and we cannot distinguish different regions easily.
    	float h = 0.0f;
    	float s = 1.0f;
    	float v = 1.0f;
    
    	float p = Math.Max(Math.Min(percentage, 1.0f), 0.0f);
    
    	if (p < 0.125f)
    	{
    		h = 0.0f;
    		s = 1.0f;
    		v = 0.5f + 0.5f * p / 0.125f;
    	}
    	else if (p >= 0.125f && p < 0.375f)
    	{
    		h = 60.0f * (p - 0.125f) / (0.375f - 0.125f);
    		s = 1.0f;
    		v = 1.0f;
    	}
    	else if (p >= 0.375f && p < 0.5f)
    	{
    		h = 60.0f + 60.0f * (p - 0.375f) / (0.5f - 0.375f);
    		s = 1.0f - 0.5f * (p - 0.375f) / (0.5f - 0.375f);
    		v = 1.0f;
    	}
    	else if (p >= 0.5f && p < 0.625f)
    	{
    		h = 120.0f + 60.0f * (p - 0.5f) / (0.625f - 0.5f);
    		s = 0.5f + 0.5f * (p - 0.5f) / (0.625f - 0.5f);
    		v = 1.0f;
    	}
    	else if (p >= 0.625f && p < 0.875f)
    	{
    		h = 180.0f + 60.0f * (p - 0.625f) / (0.875f - 0.625f);
    		s = 1.0f;
    		v = 1.0f;
    	}
    	else // p >= 0.875f && p <= 1.0f
    	{
    		h = 240.0f;
    		s = 1.0f;
    		v = 1.0f - 0.5f * (p - 0.875f) / (1.0f - 0.875f);
    	}
    
    	return HSVColor(h, s, v);
    }
    
    public static Color HSVColor(double h, double S, double V)
    {
		double H = h;
		while (H < 0) { H += 360; };
		while (H >= 360) { H -= 360; };

		double R, G, B;
		if (V <= 0)
		{
			R = G = B = 0;
		}
		else if (S <= 0)
		{
			R = G = B = V;
		}
		else
		{
			double hf = H / 60.0;
			int i = (int)Math.Floor(hf);
			double f = hf - i;
			double pv = V * (1 - S);
			double qv = V * (1 - S * f);
			double tv = V * (1 - S * (1 - f));
			switch (i)
			{
				case 0:
					R = V;
					G = tv;
					B = pv;
					break;
				case 1:
					R = qv;
					G = V;
					B = pv;
					break;
				case 2:
					R = pv;
					G = V;
					B = tv;
					break;
				case 3:
					R = pv;
					G = qv;
					B = V;
					break;
				case 4:
					R = tv;
					G = pv;
					B = V;
					break;
				case 5:
					R = V;
					G = pv;
					B = qv;
					break;
				case 6:
					R = V;
					G = tv;
					B = pv;
					break;
				case -1:
					R = V;
					G = pv;
					B = qv;
					break;
				default:
					R = G = B = V;
					break;
			}
		}

		return Color.FromArgb(255, Clamp((int)(R * 255.0), 0, 255), Clamp((int)(G * 255.0), 0, 255), Clamp((int)(B * 255.0), 0, 255));
	}
}