"What is the colour of his hair?"
Translation:Beth yw lliw ei wallt e?
An equivalent of 'the' is not required in the Welsh pattern here - the possessive ei (his) is definite enough.
Similarly, where there is a proper noun involved:
- tîm Cymru = 'the team of Wales' - usually put into English as 'the Welsh team'.
But, where there not enough 'definition' from another source, y is required:
- lliw y car - (the colour of the car) - the car's colour
- to'r tŷ - the house's roof
- tîm o Gymru - a team from Wsles
- to tŷ - a roof of a house, a house's roof
- lliw car - a car's colour