Have you noticed that when young children draw they fill up the whole paper with exuberant color? But sooner or later this expressiveness is replaced with unsure figures, often not filling the space of the paper. And by the time we become adults, many of us have been told that we are no good at art, and we may actually believe this even though as children we loved drawing.
When children draw, some draw colors around people and animals. Often a well meaning adult tells them there are no colors there, just like a child is sometimes told that they should not draw a face green or or a tree yellow.
There really is no reason to tell children how to draw. If they are interested, they can get some art education as they grow up, but there is no benefit to the young child to be instructed to draw the limited reality that adults see.
So let them experiment and don't limit their expression! Maybe then, we will have more beautiful surroundings as our children grow up and create more color in our world.