add method
Add the red, green, and blue channels of other with
those of this Color. The resultant alpha is maintained from this,
unless a strength would be specified which is used instead.
See alphaFromStrength for details on using a value between 0..1 or
2..255 as a shortcut for specifying alpha/opacity override.
Exposure method for + operator, providing strength.
Implementation
Color add(Color other, [dynamic strength]) =>
withAlpha(alphaFromStrength(strength) ?? alpha) + other;