I don't see that it would "Void the warranty" as Ruger themselves use Cerakote on the guns that are "colored".
FYI, Cerakote is tested (by ASTM methods) for salt spray durability, abrasion, flexibility, durability, and is tested and meets the military requirement (MIL-DTL-44436) for signal management for near-IR. Cerakote out performs all other firearm coatings and is the number one coating for firearms manufacturers.
Am I a Cerakote fan, yes. I've been coating firearms for about 8-10 years now, all with Cerakote and have never had a complaint about the product. Others may like other coatings and that is their prerogative, this is still a free country.