"That fish is purple."
Translation:SuD ghotI'vetlh 'ej Doq.
I would consider that more confusing -- like "it's blue and the fish is red". You don't know what the SuD refers to. I think it's clearer to put the subject after the first verb, rather than after the second one -- SuD ghotI'vetlh 'ej Doq. "The fish is blue and it is red = The fish is blue and red = The fish is purple".
SuD 'ej Doq isn't a single verb "be purple"; it's two verbs (which could be entire sentences by themselves) joined by the sentence conjunction 'ej "and".