If i can remember, bungie was trying to do a custom HUD for Reach, they where trying to do custom colors but they said it was too hard for them and they ended up giving up on the feature so they just made the original halo blue color.
I should say this here, but, Halo Reach is incomplete, Bungie was going to add a lot more things to the game, i even heard a rumor that if they would had more time to do the game they could of done space battles for online, just like the space mission on campaign but for online...