Translation:Red, white and green are colours of the Welsh flag.
Why "ydy" not "ydy'r" and why "lliw" not "lliwiau"? Would it be wrong to say "coch gwyn a gwyrdd ydy'r lliwiau baner Cymru"?
lliwiau would be fine, although lliw means 'colouring, form of colour' as well as 'colour', so the singular is perfectly acceptable in the Welsh.
y is a bit more of a poser here. baner Cymru is a composite proper noun meaning 'the flag of Wales' or 'the Welsh flag' - with no y normally needed in the Welsh. So lliw baner Cymru properly translates as 'the Welsh flag colour(s)' or 'colour(s) of the Welsh flag', which is rather clumsy in English - so it is normally rendered as 'the colours of the Welsh flag'.
The sentence is missing a comma in any case, and it has been deleted from the course for the moment, but it may not actually disappear until the next release.