Through out the Star Wars series we catch glimpses of different R2 units. Many of them are styled differently than R2-D2. Is there any listed standard that has an indication of differences in the styles?
The R-series astromech droids are perhaps the most iconic droids of the Star Wars franchise. According to "The History of R-Series Astromech Droids", an article by Pablo Hidalgo published in 1995 by West End Games as part of the "Star Wars Adventure Journal 7" supplement to their table top RPG line, the R2s are probably the most sought after droids of the Star Wars universe for many different reasons.
The first of these reasons is the fact that they were built specifically to fit in military spacecrafts slots, compared to the previous R1s that were tailor-made for government ships. This made them extremely popular in the fighter pilot circles, which in turn made them very popular with the general public, since when you'd see a holo of a heroic pilot, you'd always see his trusty R2 unit with him.
With the increased demand for such astromech droids, and the diversity of spaceships they had to be able to interface with, the R2 series were soon made to be very customizable. Although they came with a standard array of tools and sensors, it was easy to mod them to suit your specific needs, adding more tools or exotic apparel (such as jet thrusters or underwater propellers). Although I have never seen any specific reference to the R2 series' colour schemes, it seems very plausible to me that such a customizable droid would allow for custom colouring and detail work. Just as you can buy the exact same car in many different colours, I believe Industrial Automatons, the creators of the R2 series, made many different colours available for their droids.
As such, I do not believe that the colour has anything to do with the junk inside the droid's trunk. I have personally seen or read about blue, green, red, black and pink ones, and I always felt that their color schemes had more to do with their owner's taste than any hardware difference, especially since most of them belonged to fighter pilots and had what seemed to be pretty standard R2 unit features without much modifications other than the usual personality quirks from not having enough memory wipes. Usually, though, the different colours are shown in the exact pattern where R2-D2 would be blue. But, in Return of the Jedi, you can see on the Death Star a black and red R2 unit with colours placed very differently from R2-D2's (R2-Q5).
Feel free to ask about specific details if you feel this answer is incomplete. I will try to refine this answer to the best of my knowledge!
I think the color is just a matter of personal preference, on the part of the buyer. Just as you can buy a particular model of car in many different colors, so it is with R2 units