Quote - so what's the proper way to make a bumpmap?

IMO if you do want to base it on your color map, you should separate it out into various things like pores, pimples, wrinkles etc. Then you correct/invert them to the right values of bump, layer them and then test and adjust the layers till it looks ok in a render.

Or you can use only the fine wrinkles and pores of your color map as both of those are easily separated with the high pass filter. The rest of the larger details can be painted by hand. If it's not a human or you dont have a lot of refs to work from you can generate a basic noise/pore map in your texturing tool and do everything else by hand.

All depends on how much time/effort/quality you can afford of course, but you will generally get a much better result then just taking a texture and desat-ing it.