Are you confusing subtractive and additive colors? For subtractive (used for e.g. paints) you use CMY, with white being what you get with no colors and black is a perfect mix of full CMY. With subtractive each color takes light away.

Additive (used for lights) works the other way round: the base colors are RGB. No light colors is black, all light colors is white. Adding another color in additive adds more light.

So, sure, if you use additive base colors in a subtractive process, you will get garbage and vice versa.