Actually, the older form of rua had a final —dh: ruadh.

And this is entirely consistent with it being cognate with ‘red’, ‘ruddy’, ‘rauðr’, ,rot‘, &c. via IE. “reudh-”.

There was an IE word “ereb(h)” with similar meaning (thus ruby, rubricate, &c), but in this case the older texts are entirely consistent with the theory that rua is derived from ‘reudh-’.