What's the difference between "What color is this?" and "What's this color?". If someone is asking a kid, which one is more appropriate? Should he use "Which" instead of "What"?
"What colour is this?" Can have two meanings.
- What is this colour called?
- What colour is this item?
"What's this color" only has the first meaning. "Which colour is this?" also only has the first meaning.
"Which's this colour" is not good English.
So use the least ambiguous, most accurate option. That's most appropriate for anybody, regardless of age.
Of course, the answer to either form of the question will be the same.