Best thing to do is get an empty coke bottle (1L version) and cut it in half along its length (cut the top bit off first its easier to do).
Then try spraying the paints onto the inside of the bottle, (clean it first
).
This way you will be able to find out if it looks better with the purple going down first or the red going down first.
Better than ruining a shell.
As jon said, when it comes to doing the job propper, warm the cans up first. It will produce a finer spray.
Remember to back them up with white when you're done, it will look brighter.