What two colors makes mustard?

I believe, you mean what two colors make yellow?

Well, none really, because yellow is a primary color.

A primary color is a standard color, the primary colors are: red, blue, and yellow.

Every color in the universe would start as a primary color, such as: blue and yellow make green or blue and red make purple.

No two colors can be mixed to make a primary color, it just doesn't work.

So, frankly, the answer to your question is none.