Whether by scanning or photography, artists generally do their best to reproduce the colors of their paintings as accurately as possible. They may also correct and enhance colors digitally, but the goal should be to match, not improve upon, a painting’s original colors. Unfortunately, there can be a great deal of difference from one monitor to the next in how paintings appear online. My scanner and monitor are fairly well calibrated, and I always check images I post on my iPad, which displays pretty accurate color.
Most paintings look best in good light, but you should not have to display a painting in direct sunlight to see the right colors. If you have purchased other paintings and been satisfied that the color displayed online was reasonably accurate (so you’re convinced the problem is not your monitor), and you think the color of this one was not accurately represented–and you’re unhappy with the original–I think it would be fair to return it, and say why. If the artist digitally enhanced the photo, it should be a lesson. Most artists don’t want people to feel they have to keep something they are not happy with.