Get the color given a ratio between a start and end color. From: http://www.hjerpbakk.com/blog/2015/2/15/gradient-colors-on-ios

ColorUtils.cs

public static UIColor ColorForRatio(float[] startColor, float[] endColor, float colorRatio) {
	return UIColor.FromRGB(startColor[0] + (endColor[0] - startColor[0]) * colorRatio, 
		startColor[1] + (endColor[1] - startColor[1]) * colorRatio, 
		startColor[2] + (endColor[2] - startColor[2]) * colorRatio);
}