I thought it was Red, Blue, and Yellow.

That's what they teach in elementary school, but they're talking about paint, not light, when they teach you that. And it's still not quite exactly right. Technically it would be closer to magenta, cyan, and yellow, but they don't want to teach you words like "magenta" and "cyan" in kindergarten.

Each of these three basic pigments absorbs one of the major colors of light that the eye sees. (Of course, the cheap paints you work with in gradeschool don't do so perfectly, by any means, but this is how it's supposed to work. If your pigments were perfect, mixing them all together would give you black; in practice, you usually get a murky brownish color, because the pigments you have are imperfect.)

For instance, yellow pigment absorbs blue light, so what gets reflected is mostly green and red light; when the eye sees green and red, the brain interprets it as yellow. The other two pigments work similarly. For further reading on this subject, see CMYK color model.