It's pretty incredible that in 2011, with all the available technology, we don't have a way to accurately represent the colors of a painting.
When you take a photograph of a painting, no matter how advanced your equipment, you have just removed the color one step from its source. From there, the gap only widens as you view the image digitally on a screen, even one that is color calibrated, never mind printing it out. Now you involve processes of translating digital information to ink on paper, or in the dark room, color technologies contingent on the film you used, the paper you print on, the chemicals, the timing, the choices you make.
You only have to glance at a reproduction in an art book, even a very expensive one, and compare it to another book; compare either of those to a postcard from the museum that owns the piece, and compare any or all of these to the myriad digital images you'll find in an online search, to see that color is entirely fugitive, subjective and subject to both the limits of technology and the vagaries of interpretation.
The only way to know the color is to stand before the original and behold it with your own eyes. Even then, as soon as you move away, you begin to forget...and no postcard, book or online reproduction, no matter how credible the source - even the museum's own website - can be trusted to show you what your eyes perceived.