Other than the *obvious* red Gyarados, I've caught a purple drowzee on pokemon gold. Seen that one a bazillion times...
Other ones I've seen on pokemon gold and crystal are blue gastly (I've also caught that one a bunch of times too), green zubat (didn't have any pokeballs!!!), and yellow caterpie. There's probably a couple more that I am forgetting...
On pokemon ruby (or was it emerald?), I caught a yellow poochyena twice.
Fire red I got a shiny (I think it was yellowish) spearow and a yellow caterpie.
On platinum and pearl I've caught a pink purugly and a light-colored bidoof.