cooler method
Adjusts the hue of this color by amount towards
270 degrees, capping the value at 270.
If relative is true, amount will be treated as a percentage and the
hue will be adjusted by the percent of the distance from the current hue
to 270 that amount represents. If false, amount will be treated
as the number of degrees to adjust the hue by.
Implementation
@override
HspColor cooler(num amount, {bool relative = true}) {
assert(amount > 0);
return super.cooler(amount, relative: relative).cast();
}