Well, according to the R2 Builder’s group, who have extensively researched the paint colors and who have had access to the original R2 costumes and remote controlled props, the blue color for the ANH R2 had a purple base that made the blue color more cobalt blue than royal blue. That somewhat changed for the TESB and ROTJ, but the color only changed slightly, keeping a touch of the reddish hue.
These two reference photos reveal that there was a lot of red in the rocks, skin tones, and how dark R2 was on set.
Compare that to DrDre’s correction:
