Don't be afraid to experiment, the computer is a wonderful medium for altering images and changing colors. Here is an image consisting of two figure studies which I will use as an example of how I typically experiment with color and design.
Here is the basic image that I started with.
First I just added a yellow color.
Next I made a 50% copy of the original and positioned it over the larger image and added tone of orange.
Here I placed a 75% image over a negative image and added a few more colors.
In this version I tried a whole new color scheme.
In this last version I added a dark brown and a light purple color tones, again changing the whole look of the piece. I could keep going on, of course, as it's quite easy to do this type of thing on the computer, something that would take days or even weeks to create using traditional mediums such as acrylic or oil paint.