I've got some Images like this one:
Now I need to find out the color which will become the orange in the right center of the image when drawn on a new layer with 50% opacity (The Background is the grey color).
Sadly I've no idea how to do this.
Try this one:
I tried this with two colors. I think Photoshop uses transparent tool with a polynomial function, not a linear function. Probably because of that, first test gave me a result accuracy of 96%. Second test gave 99% accuracy. I hope it helps.