The need for a black R2 was a byproduct of issues with primitive bluescreen techniques of the time. Because of the nature of the automotive paint used on him (at the time, a dry, pearl application of candy blue topcoat over purple basecoat, which is extremely dark in the flash (all but dead-on angles)) it isn't possible to simply color him in those shots. You would have to digitally replicate the complex paint and track an R2 head to the original to derive new photographically accurate dome panels which could then be transferred onto the original. During my work creating renders of R2 for the Star Wars Complete Visual Dictionary, I was sent to the Ranch to study a few of the original R2's and was able to perfect my CG R2 - including a physically-accurate digital simulation of the paint, rendered in a next-generation software engine which correctly reproduces the electromagnetic wave propogation of light.

The long short of this, of course, is that the Legacy Edition was able to benefit, and features as accurate a coloring of the dome panels in ANH as you could possibly hope for, shy of refilming the sequence. At some point, I'll post screenshots...

_Mike