Hi-temperature exhaust paint is what you want. There are several proprietary brands out there and they are all OK. I have used the stuff from PJ1 and it gives a good finish.
There is no easy way to do the job properly and if you want anything like a good (or effective) finish you will have to remove the pipe from the bike. You will need to remove all loose paint and rust from the pipe. Start with a wire brush then smooth off with wet & dry. Before you paint clean the exhaust with white spirit to ensure that any last trace of grease or other contaminents have been removed. Keep the paint indoors before using - you can't spray properly from a cold can. Mask of any threads. Use even strokes along the length of the pipe and build up several thin coats to get good coverage. (Don't try and save time by applying one thick coat - the paint will be uneven and will run and end up looking shite!)
I know you want a quick fix but time and a little effort are needed for a decent finish.