If you ask this ques then ans will be like 3 or 7 or infinite......
In computer there are 3 basic colors (R,G,B) which ranges from 0-255 i.e. for each color 256 color pattern exist. So
Total No. of colors = 256 * 256 * 256 = 16,777,216
As per the website http://www.cis.rit.edu/fairchild/WhyIsColor/Questions/4-4.html the color in viewing condition is infinite.