If you absolutely must have the 4pc bonus you can always rip the mods out of the chest and use your favourite orange chest. You still get the 4pc and the chest is the "main" look anyway. This is what I did on my assassin so I didn't have to have the hideous Tionese look. By the same token, it's the same if you only want the 2pc bonus too, just pick the bits that fit best with the look you want and keep those.
And if one of those pieces is the hat, you can just hide it. Boots are also completely hidden if you're wearing robes, and on at least some sets are the least worst looking piece. Pants are also usually covered up with other parts, and don't tend to have that much detail to interfere with other pieces when you unify colors.
Many of the set bonuses don't make that much difference in practice, even if they are nice to have. If you have appropriate mods in the gear for what you're doing, then for all but the hardest endgame encounters, if you fail or wipe, it won't be because of the lack of a set bonus. And by the time you get to where the margin of success is that tight (if it ever is at all), you'll be in full Campaign anyway.