Explanation
I have a semi-transparent color of unknown value.
I have a sample of this unknown color composited over a black background and another sample over a white background.
How do I find the RGBA value of the unknown color?
Example
Note: RGB values of composites are calculated using formulas from the Wikipedia article on alpha compositing
Composite over black:
rgb(103.5, 32.5, 169.5)
Composite over white:
rgb(167.25, 96, 233.25)
Calculated value of unknown color will be:
rgba(138, 43, 226, 0.75)