When all the colors or wavelengths of visible light are present, they combine to give white light. Note that the rules for mixing colors are different for light and for paint. Mix red, green, and blue light, and you get white light. Mix red, green, and blue paint, and you get brown paint.
That's because paints and crayons do not absorb and reflect pure shades of color — there is always some mixture. See the difference between mixing light and mixing paint colors.