White light has all the different colors in it - that's why a prism or water drops can make a rainbow by making the different colors come out at slightly different angles. When you have, say, red paint, it's reflecting the red light and absorbing all the rest. Same for any other color. So when you mix paint, you're mixing stuff that absorbs different parts of the light, the more different ones you're mixing in the more you're absorbing. Mix all the colors and you get a dark grey going on black because it's absorbing most of the light.

But with computers, you're not mixing things that absorb light, you're mixing the light itself. If you mix all the colors of light together, you get white (like doing the reverse of making a rainbow). So the formulas for making colors are kind of inverted from mixing paint.