From the FAQ at this website

RUDE RAINBOW?

We're often asked about the so called rude Rainbow, clips of which have been shown on the Channel Four programmes, TV Unzipped and TV Offal. Although often introduced as the unaired pilot, this episode was actually specially made by the team at Thames Television. Geoffrey Hayes told us, "The VT department of each ITV company, every Christmas had a competition to enter all the outtakes from their particular companies programme out put, and see which ones were the funniest. But ours wasn’t an outtake, we specially did it. It was specially scripted and we did it and I never saw it! We did it and Thames won it, but what they actually won, I don’t know. But I never saw the video. I can’t remember much about it except for Zippy with a banana." The script was written by the voice of Zippy and George; Roy Skelton. Sadly, we don't have a copy, so don't ask!!

I haven't watched the clip yet, but reading between the lines, I guess this is it.

Edit - oops, didn't read right to the bottom of the link Roger posted, where it says more or less the same thing