Different color when combining layers

Hi. I have a problem that I do not know if it is a mistake of mine or of the photoshop. I am changing the color to a black and white image. To achieve this effect I have put 2 layers with colors in "overexpose color" mode. When he shows it to me on the screen, the color I want appears, but when I combine it, another color is seen.

What can be the failure?
Sorry for my English