Direct sunlight will quickly destroy or 'bleach' a watercolour or print, removing the 'red end' of the spectrum from the image's colour and leaving it with a blue tint. The more intense the sunlight and the more long-term the exposure, the more pronounced the blue tinge and the less visible the reds.
This is something easily avoided, if you look on eBay, you will frequently see such damaged prints and paintings, with blue casts. It is also something that is only gradually is noticed, meaning that by the time you really realise the damage; it is already far too late.
All artwork, including the most valuable, is similarly vulnerable, so an expensive limited edition print or original watercolour will not be immune. This damage is also irreversible, so not only will the colours be permanently be destroyed, but the value will additionally have been similarly wiped out.
To avoid these problems is really simple – place your artwork on a wall away from direct sunlight and when storing any unused artwork, ensure its reverse faces outwards and the art itself faces the wall and not the light source(s). If you have a particularly valuable artwork, the ideal viewing location to situate it is often on a stairway or landing, which tend to be free of any direct sunlight, throughout the day.