Many people get confused between the primary colors of pigment and the primary colors of light. To keep these straight we must remember that pigments reflect colors to our eyes, and they are subtractive; if we mix two colors they will each continue to absorb the wavelengths they absorbed separately, so fewer wavelengths will be available for our eyes to receive. The ultimate end of this process is black, where a combination of pigments absorbs all the colors are we are left with no wavelengths being reflected. Red, yellow and blue are the three primary colors of pigment, which can produce black.
In the case of colors of light, the process is additive - two or more colors of light added together increases the number of visible wavelengths. The primary colors of light are red, blue, and green. These three, when combined, make white light. There are also three secondary colors, which are yellow, cyan, and magenta - these are produced by a combination of two of the three primary colors.
Since the primary colors of pigment are red, yellow, and blue, and the primary colors of light are red, green, and blue, the answer to your question is yellow.