Well, after reading your thread and posts regarding Joey and Colin, it sounds as if you didn't introduce them gradually. Cats are creatures of habit. Introductions need to be made, not just throwing Joey in and hoping they get along.

There have been MANY threads on how to gradually introduce cats and keeping harmony on the kitty front.

Here's some advice I gave on one of the threads:

The way to create a happy environment is to introduce them gradually. Keep him in the bathroom and rub him down with a towel. Allow your cats to smell his scent. If you can, do the same with your cats and allow "new" kitty to smell their scent. If you can get baby gates from friends, stack two of them, on on top of another, in the doorway of the bathroom, allowing them to see each other, but having protection. There will be hissing, spitting and growling. After a week or so of that, let the new cat out to explore, putting your cats in a room. Allow new kitty a while to get used to his surroundings. Show him where the litterboxes, food and water are. As long as there is no bloodshed, just standby with a squirt bottle to break up any real confrontations (ears back, fighting stance, bloodshed)

I think the right thing to do is to get one of them to the vet (if you think it's Colin, take Colin) to rule out a UTI.