Today’s painting software has a significant flaw: colors do not mix like actual paints.
For instance, blue and yellow make gray instead of green.

This is because the majority of painting software is built around the RGB representation.
RGB simulates the mixing of colored lights. Paints, however, get their color from pigments.
Physically-based mixing of colored pigments is predicted by the Kubelka–Munk model. However, implementing it into any painting software has been too impractical because it required to leave the RGB representation behind and to rebuild the core structure of a program. That’s the main reason why Kubelka-Munk has never been adopted by painting software in practice.