Red-green and blue-yellow: colors that exist but we can’t see them.
A red-green color exists and it is not brown or what you see when you mix the pigments together. Also similarly, yellowish-blue is not green. If you try to imagine these colors, you probably can’t since there’s no point of reference in your mind. But they do exist and are called “forbidden colors.” We just can’t see them and this is normal because of the specifics of human color perception.