////https://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef
public static double LuminanceRatio(Color c1, Color c2)
{
//(L1 + 0.05) / (L2 + 0.05)
Color L1, L2;
if (c1.GetBrightness() > c2.GetBrightness())
{
L1 = c1;
L2 = c2;
}
else
{
L1 = c2;
L2 = c1;
}
double ratio = (L1.LuminanceValue() + 0.05) / (L2.LuminanceValue() + 0.05);
return(Math.Min(ratio, 21));
}