Robo, even though he had the serial number R-26Y, was not a member of the R-Series.
Ok, maybe I should restate that a bit. Given the R, he might have been an R-Series model, but his function was far different. Unlike the rest of the R-Series, who's main job was the guard the factory, his was to befriend humans and lead them to their execution at the Geno Dome. He was build much differently as well, and had gadgets and was stronger than the other R-Units.
This is portrayed in the fact that he actually had a name besides his serial number: Prometheus. Him and Atropos were the only two given a name by Mother Brain, and that's mainly because they served the same purpose.
To put it shortly, Robo had a different paint job because he was a higher rank than the R-Series units you fight in the derelict factory.