In Minecraft, how do you get sheep named "jeb" to return back to their normal colour?

First of all, you could just get a new sheep. Looking at your question, that probably isn't it, but might as well get it out there. Unfortunately, you cannot change the sheep back to normal without using another name tag. Since the reason of the color changing is the name, it cannot be reverted without another one.

Dyes will not work on the Rainbow _jeb sheep, so giving the sheep another name tag, (naming the sheep "Dinnerbone" will work, but it will flip the sheep upside-down, like the name does for any other mob) will revert it to its original color.