Do these sentences have identical meanings and are they grammatically correct?
- What colour is her hair?
- What is her hair colour?
The question asks is "the colour (of her hair)" the same as "her hair colour"
Normally these would be equivalent. Sometimes, in context, "hair colour" might mean the same as "hair dye", so it would be possible, in context, for "What is her hair colour" to mean "What brand of hair dye does she have?". The first expression is the normal way of asking about someone's appearance.