What color is not considered a primary color of light?

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.