"What colour hair do you have?"
Translation:Pa liw gwallt sy gyda ti?
sy is a special form of bod which is used instead of mae when the meaning 'which/who is it that...' is needed.
Here, because of the pattern using gyda for possession, we need to say the Welsh equivalent of pa liw gwallt is it that is gyda ti?:
- Pa liw gwallt sy gyda ti? - What colour hair do you have
- Pa gar sy gyda ti? - What car do you have?
- Sawl brawd sy gyda ti? - How many brothers do you have?
- Beth ydy enw'r ci sy'n eistedd wrth y dyn 'na? - What is the name of the dog that is sitting by that man?
I think I get it... but could you use yw or ydy instead though, or does Welsh, unlike English, require you to say "What colour hair is it that is with you?" rather than "What colour hair is with you?"?
That is slightly different pattern in both Welsh and English - 'What is the colour of her hair?' vs 'What colour hair does she have?'.
The purpose of the question is the same, but the phrasing is different.