contrastRatio method
Calculates contrast ratio with another color
Implementation
double contrastRatio(Color other) {
final l1 = computeLuminance();
final l2 = other.computeLuminance();
final lighter = math.max(l1, l2);
final darker = math.min(l1, l2);
return (lighter + 0.05) / (darker + 0.05);
}