What does red blue make?

green

Somebody was joking here. Usually they make black or a very dark purple.

However, I have seen red and blue produce green. The blue was Prussian blue, which does pass green. And the red was an oil paint version of chromium red, highly diluted to the point where it was yellow orange. Both pigments passed green.

If we are talking red light and blue light, instead of pigments, they make magenta.