Riso inks are mostly translucent. This means that when you print one ink over another, the colours interact in interesting ways.
This is one of the best things about Risograph printing. With clever overprinting, a two-colour print can look like it
contains three colours.
Try the screen simulation.
This is a general feel of ink mixing.
The exact Risograph result are textured and depend on the inks, paper, ink density and the order in which colours are printed,
so overprinting is best treated as part of the character of the process rather than an exact colour-matching system.