[EpsilonX]

I think they tend to go with common colors that are bold and have a lot of "pop". The standard Red, Blue, Yellow, Green, Black, White, and Pink all contrast each other well and make for a very vibrant looking team. Orange and Purple both look nice too, but they aren't really as popular. Silver shows up sometimes too, but I personally think Silver rangers usually look kind of bad, so...

The colors I listed are tried and true, so I don't think they'll really be changing it, unless they're purposely trying to try something new. To be honest though, I don't see that happening.

As for themes, the suits usually reflect the theme of the show (dinosaurs, ninja, trains, etc.) but that's about it. Each season has its own theme to differentiate it from other seasons. There doesn't really seem to be any sort of pattern when it comes to the themes though, so they likely just go with whatever sounds good or works with the plot they've written. Or something completely different, I don't pretend to know how the writing process works at Toei.