GTBO55, you can actually use pearl both ways!!
the difference is in the result! if you add it to a base coat clear (no gloss) it will come up alot more prominant and look like its over the top!! when you add it to the paint, it takes alot more pearl to get as strong an effect, but is easier to match if you know the ratios!!!
as to adding it ontop of another colour, if its durin the respray it will be easier and cheaper, i paid about $100 for a litre of PPG gold pearl plus the litre of base clear for about the same, then add thinners and hardener!! but i still have about 750ML of pearl left for my next project
My paint job has 1.5 coats pearl with clear and 2.5 coats normal clear!!! half coats being mainly a dust coat of the surface!!
there are heaps of pearl colours you can choose from, so get the paint shop to show you a few!!