You have three primary colors: Red, Green and Blue. There are three supplemntary colors in between the primary colors. red, YELLOW, green, CYAN, blue, MAGENTA, red. THink of them on the edge of a wheel. Now filters will allow light of similar wavelength to pass through while blocking all others in increasing degree the further around the wheel the color gets from the color of the filter. So a Red filter will raise the values of reds in your subject in relation to the other colored pigments from which light reflects. Since it allows reds to pass through, your blues, green and cyans will drop off darker in relation to other colors. Go to my website in my sig and hit the 'Better Photography' tutorial and then choose the chapter on color and light. Lays it all out. Hope this helped.
Chris