Why is green a primary color for light?

Red, Blue, and Green combined creates a white color.

You are correct, but that is if your mixing pigments to make a hue. Red Blue and Green are the primary colors of light.

If you pass any non-primary colour through a prism it splits it into its constituent colours

e.g. white light splits into the familar spectrum of the rainbow.

if you pass “mono-chromatic green” light through a prism it does not (and cannot be) split into any constituent parts. It is a pure, “single frequency”, (alternatively - “single wavelength”) colour.

– Best wishes - Majikthise.

True but he was talking about light not pigments

@Daviesha Hubert EXACTLY and it was YOU that changed it to comments about pigments (whether you realized it or not).

I quote the following from your answer: “… Because when it is mixed with 1 of the other primary colors (red and yellow) it makes a secondary color “ …..

…. But YELLOW is a SECONDARY colour that is formed by adding RED light to GREEN light (when talking about COLOUR ADDITION),

and both funnyboy56 and the OP epiclygreen were perfectly correct in their comments - they were talking about COLOUR ADDITION where LIGHT is added.

You are talking about COLOUR SUBTRACTION which occurs when pigments (paint) are mixed, but you wrongly state that green is a primary colour in such circumstances - GREEN (paint) is formed by mixing the two (primary paint) colours YELLOW and BLUE.