Alright ... so the shuttle, orbiting a measly 17,500 miles above the daylit surface of the Earth gets enough light to render the color blue. What about an X-wing passing under a red gas giant, or dogfighting over a gray metal sphere? I think we've usually seen starships within the well-lit band where habitable planets lie, but I do wonder to what extent nearby celestial objects would influence their coloring.
(Special Edition answer: It makes them blue!)
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Outer space scenes in Star Wars were filmed in the studio using blue-screen at (an über-measly) 0 miles above the surface of the earth (NASA 1 - Lucas 0) (insert smiley)
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The use of blue screen explains R2's blue to black color change. (Note that markings in other colors are not affected by the use of blue screen.) For instance the red markings on the X-wings or the yellow markings on the Y-wings don't fade to reddish brown, black or grey.
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Theoretically speaking, if red screen would have been used instead of blue screen back in 1977, R2 would never have changed color but the markings on the X-wings would have.
In the special editions the "shortcomings" of an old technology are corrected (*), although not in a consistent way as ep. 4 shows us.
(* I'm not implying this was nessesarily a good thing to do , just stating a fact.)
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Now the Death Star battle in Star Wars takes place next to a big red planet (Yavin). Both this planet and the Death Star itself are well-lit so there is no reason to assume that small X or Y-wing fighters would not receive an equal amount of light.
Also when light conditions are low , all colors across the spectrum will start to fade, not just one. (There will only be marginal differences between colors - mostly red will fade first - yellow last.)
In other words if a low-light space ambience was desired all colors and brightness (Yavin, Death Star, X-wing, Y-wing, etc...) would have been toned down considerably, not just poor R2's blue dome.