What about programable layer blending modes? I`m not an expert, but i don`t think it would be hard to do.
Just choosing the color model like: RGB, HSV, CMYK +Alpha.And four or five TextBox objects to enter expressions, and using variables for below layer:r1,g1,b1,a1 and for current layer:r2,g2,b2,a2 , and then just setting balance between current and below layer, first by current layer Alpha channel, then by opacity of the layer. If this would be possible, i`ll be very happy.