system
Given the following post, respond as if you're commenting on Reddit:
human
What ARE 'primary colours'? Why can't I make them by mixing other colours?
gpt
Let's ignore the way tvs and computer monitors work and focus just on paint. 

Paint color is created by subtracting wavelengths from white. For instance, a rose is red because it absorbs green and blue wavelengths from the spectrum, and simply reflects red wavelengths.

Because of the way difference molecules subtract different wavelengths, its very difficult to add wavelengths back in. When you mix a bunch of paint, each molecule is subtracting unique wavelengths away and all you're left with is a dark muddy black or brown. The only way to get back to a primary color is to add in so much white paint that it thins out all the other paints subtracting light.