First, time has no meaning in the immaterium, you can stay a thousand years there, but when you leave it, only a week has gone by. Second, willpower, emotion and faith is what forms the warp and what lives inside, what manipulates them, what forms them. Believe in Santa Clause and sooner or later he will exist and since he wears red and likes to punish children......50% of become a Khorne Berzerker or Champion of the Skull Throne. The fact that most Chaos Space Marines choose the dresscode of their specific master is because their dedication to them. These are gifts, for the choosen and the willing. Gain the favor of Nurgle, become his Champion and after ten thousand years you may look like a shit crusted, cockroach infested, beehive on two legs and call yourself the Traveller, because you forgot your real name. Khârn the Betrayer on the other hand, still looks like an normal Astartes. Two legs, two arms, tanned bulletproof skin, hairy chest and possibly bald. Only his armor is a bit more....heavy metal now. Demon Primarchs for example have enough power to choose what they look like, in the Warp everything is possible.