Color is going to depend on you, and you only. If it's in it's holster 99% of it's life, this shouldn't be a big internal debate.
If your water and body chemistry are that corrosive to firearms, the slide is the least of your concerns. You need to keep an eye on the springs, pins, and set screws on the gun. The slide will work fine even if it is pitted and covered in rust. However, small springs weaken and fail when oxidized in the micro-climate known as your pants. So please don't overlook these items when addressing the slide...
As far as finishes, cerekote, like anything else depends on your prep work. Going for a "rough" treatment may result in a quick wearing finish. Personally, I like hard chrome as a hard use finish the best, but that might be a bit more than you are looking for. Maybe birdsong black-T or robar' comparable product would work?
Finally, if your slide is rusting, also rotate your ammo.
Also, post pics when done.
. We like to see third gens