Photoshop CS5, how to create a layer mask from a layer?

I have a layer in Photoshop that I'd like to use as a layer mask, is this at all possible?

It's a black & white layer with some gradients that I would like to apply to another layer that's a solid color, but I can't figure out how to just edit a layer mask to copy and paste the image in.

P.S. I have Adobe CS5