Early games did take advantage of CGA composite color hacks to achieve 16 color graphics. E.g.:

http://en.wikipedia.org/wiki/File:Microsoft_Decathlon_RGBvsC...

http://en.wikipedia.org/wiki/File:KQ_CompVsRGB.png

However, the era of CGA composite color graphics was very short-lived. By the time the IBM-PC started to become a popular home computing platform, 16-color Tandy/EGA graphics and RGB monitors were the norm. Any composite color hacks would be rendered obsolete and unusable on these later machines.