European ski slopes also have black.
Mountain bike trails are also green, blue, red and black.
Since this project is for younger ages, I'd skip the black.
Such sporting activities as skiing, biking and others also make use of shapes to further differentiate, rather than using colours alone - thus appealing (or not) to the subconscious.
So for example a green circle is curvy and friendly; the blue square is not curvy, and the red triangle is very pointy and much less friendly (while black diamond is both pointy and absent of colour making it the least attractive).
On the other hand, having asked my own children what colours they would use, they did say green yellow and red because that was what they were used to at school (they use traffic light style symbols at school). Neither of my kids suggested blue, but when prompted, thought blue would mean hard. Neither were put off by the colour red meaning a warning - for example thinking it should be avoided.
Then my younger boy said he would use food rather than shapes - so here's what my limited research at home suggests. I chose fruit, despite my boy suggesting cucumber for easy, bread for medium, and ketchup for hard!!