With a truckload of images to finish you don't often have time for extended procedures, so here's some quickie ways to get a HUGE amount of control.

Since the Channel Mixer has been mentioned and illustrated in links (what a hottie in the first example - hubba hubba!), I will dive into the other 2 methods mentioned Hue/Saturation and Gradient Map.

Here's #1 - the Gradient Map - when you create a Gradient Map by default it will be black and white. As Inshaala mentioned, you can move the sliders around to adjust contrast. Here's another tidbit - make a Hue/Saturation Adjustment Layer between the Gradient and background image. Adjust individual colors to alter certain areas/objects. You're not limited in any way to just ONE Adjustment Layer, and it comes with a Mask (the white box to the right in the Layer Palette). So mask off what you don't want it affecting, and make another one just for new areas to affect.