The Tamiya pearl white is slightly transparent and it will take on some of the tones of your primer coat. If you use grey, for instance, the color takes on a muted, slightly grey tone. If a purer white is what you have in mind, then use a white undercoat. Tamiya's white primer works pretty well, but the primer often needs a little rubdown to smooth it out before applying the pearl paint.
Tamiya's pearl clear is alright, but not an absolute requirement. It doesn't add much more pearlescense to the finish and Tamiya's gloss looks just as good to my eye.
If you're in doubt, then test the primer/paint/sealer combinations before you go to paint your project.