composite.py

from typing import List

class Color(object):
    def __init__(self, r: float, g: float, b: float, a: float = 1.0) -> None:
        self.r, self.g, self.b, self.a = r, g, b, a

    def __str__(self) -> str:
        return f'Color({self.r}, {self.g}, {self.b}, {self.a:.2%})'

    def __repr__(self) -> str:
        return self.__str__()

def mix(a: Color, b: Color) -> Color:
    Ao = a.a + (b.a * (1 - a.a))
    Co: List[float] = []

    for Ca, Cb in zip([a.r, a.g, a.b], [b.r, b.g, b.b]):
        pre = (Ca + a.a) + ((Cb * b.a) * (1 - a.a))
        Co.append(pre  / Ao)
    
    return Color(r=Co[0], g=Co[1], b=Co[2], a=Ao)


bg = Color(32, 34, 35)
overlay = Color(124, 123, 123, 0.65)
c = mix(overlay, bg)


# Color(32, 34, 35, 100.00%) + Color(124, 123, 123, 65.00%) = Color(135.85, 135.55, 135.9, 100.00%)
print(f'{bg} + {overlay} = ')
print(c)