Dirk is right...it's all about 'depth' perception.
Basically on a tangerine with metallic or pearl, it's more of a 2D effect. (WYSIWYG)
If you go with a tri-stage candy, you with more of a rich look than the 2D look.
Slang term "Candy" (candi, kandi, kandee...) is nothing more than a clear with 'dye' in it. So when you coat a metallic, you are actually seeing through the candy into the base metallic, thus giving more depth and a nice rich look to it.
Normally candies are painted over brighter metallic colors to bring out the richness even more.