Like Keyser said, he shouldn't really where a "costume" in the period between his exile from the Corps and his acquisition of the yellow ring. However, once he gets the yellow ring, then he should obviously have the yellow costume. Skip the blue entirely. Sinestro finally has a good look, so they should use it.
However, I think there should be a period where he has the ring but not his own Corps; the idea to create a Corps should be a revelation that comes to him after an initial defeat, or something like that. I think it would be really cool to see him discover this yellow wavelength of power at the edge of the universe (sort of the way Kyle discovered the nature of Parallax in Rebirth), and build the ring himself. Following that, he'd impulsively rush back for revenge, not thinking things through. Then, after his defeat, with a cooler head he would realize that he has the resources necessary to build his own army. Alternatively, his initial solo attack, instead of being about hot-headed revenge, could simply be driven towards releasing Parallax from the central battery, thus giving him the power source he needs to fuel said army. That would be the second movie; the third might be what Blackest Night appears to be--Hal and Sinestro joining forces against a superior enemy.
I also hope they don't cop-out and change his physical appearance in order to make him less "silly." It's true, he has often looked ridiculous in the comics, but in the last few years artists have come along who have made him look really intimidating. The way he's drawn now--uniform, body language, lighting, and even his Hitler haircut--makes him look like a really scary, ominous enemy, instead of like a pink Mr. Spock. I think it's be demonstrated quite clearly that you can make him look awesome without making him less silly... if that makes sense.