If the NxN samples in a grid have different colors (e.g. 2 are red, 2 are yellow), would the overall color be a mixture of those different colors?
Yup! You just average the colors (as RGB vectors), and you'll have to do this in assignment 1. This is a reasonable thing to do because of how color is perceived: every color can be constructed as a linear superposition of three primaries R, G, and B, and mixing colors physically corresponds to averaging the RGB vectors.