Paint Transparency

I want to paint some models and use the alpha channel for a bump map.
is it possible to assign a colour a specific alpha value?

for example i want to paint red with a alpha value of 0.75, Yellow with alpha 0.1, etc…

or is there at least a way to paint on two different images at the same time with different colours?

Uhh, no. You might consider trying to control alpha in nodes, by getting the hue of your texture image and saying what hue corresponds to what alpha (here, via an RGB curves node):

You can bake that, of course; you can also make an alpha channel out of a hue channel in something like Photoshop, if you’d prefer.