In the last episode Garou gets orange hair. My question is why did Garou his hair color change to orange?
This is just part of how Garou gets progressively more and more jacked up as the arc progresses, but yet he keeps on trucking and getting stronger. It's really hard not to understate the sheer level of worked-over this guy achieves. The following happens to him in chapter 68 of the webcomic, for example, and this is after already having suffered numerous injuries in preceding battles:
The orange/red you refer to is just blood. A blood vessel has popped in his eye and he's bled so much that it's staining his hair. More clearly, you'll notice that immediately before the reveal of his red hair and eye that he still has white hair, and then rapidly wipes his hands in his hair. He's had a lot of blood on his hands and arms, bleeding notably out of them at least since deflecting Death Gatling's attack; we also see he's still rupturing blood all along his arms when he squares up against Bang.
The animation is slightly misleading after this, as you still see a far shot of him with white hair after this before the close-up reveal, nor does it show any color changes while he's doing it. I think this was all done for the purpose of the dramatic reveal being a dramatic reveal. There's no other reason for wiping his hands on his hair other than to transfer blood to his hair, either strictly for a makeshift cleaning of his hands or, more likely, because he knows it serves to make him seem more monstrous.
In the webcomic, by chapter 82 he ends up a full-on horror freak show from the battle damage alone.
He does later on achieve what appear to be legitimate monster-like transformations, but at this point in the story, if the manga/anime aren't going to seriously alter the nature of his progression, then all you are seeing is the accumulation of (severe) battle damage.
In his path of becoming a monster, his appearance changes. First, one of his eyes turn red and also his hair, later his clothes merge with his body, later he gets horns, finally he gets bigger and an even more monster-like appearance. According to a Youtuber with over 2 million suscribers which covers mainly One Punch Man and Dragon Ball, Garou has 6 forms (detailed in the video)
Gyoro Gyoro (one of the leaders of the monster association) says near-dead experiences help humans turn into monsters, and that was her plan for making Garou even stronger than the monster king Orochi.