Just play around with it on some scrap pieces.
The idea behind a metallic undercoat is to give a slight "shimmer" effect to replicate fine cloth. This can also be handy for painting more realistic hair.
Just thin down your top coat colors more than you normally would and apply light layers until you build up to the effect you want.
The "pearl" finish (over coat) kind of does the same thing. Again, play around with it and try to keep it subtle. Paint some test samples then look at them again the next day. If its too strong, try again and tone it down some more.
Imagine you want to paint a tartan shawl. Its a rough, hand woven piece of cloth. Paint the basic colors then lightly add a thinned down "pearl white" color on top. Shazam, the cloth is now older and more tattered/frazzled looking.
You can search on YouTube for folks in Japan who build Gunpla characters and Bandai figures in some larger scales. They have some "out there" ideas about basic figure painting (ie, flesh tones) but there's always something new to learn and apply to your own corner of the hobby.
