The easiest way to color a black and white sig is by using color balance layers. On the layer palette, there is a series of buttons across the bottom, one of which is a circle that is half white, half black. If you click on that button, that brings up a list of adjustment layers that you can create. Choose "color balance", and play with the sliders you see there to color your sig. Check out, and search for tuts on coloring sigs to get additional hints.