What it teaches
That colours are made from other colours, and which ones. Red, yellow and blue make every other colour on the palette, and that is genuinely surprising the first time.
It is also a child's first experiment with a rule they can test themselves, with paint, five minutes later.
What to look for
Do it with real paint afterwards. The screen is a rehearsal; the mess is the lesson, and a child who has mixed brown by accident will never forget what happens when all three go in.