I found out a while back (much to my consternation) that the leather's exposure to the "daylight" bulbs will cause the dyes to fade (especially blues, purples, and greens), just as if you left them in the sun. So I have been recommending to my customers, if they are going to display my work in their home or office, to use the soft white bulbs. I believe the reason for this is that the daylight bulbs emit more ultraviolet radiation - they are more "blue-ish" - which is the type of radiation we get from sunlight that causes dyes to fade quickly.
What I don't know for sure is whether the same thing that causes dyes to fade is also what gives leather that suntanned look, but I'm guessing it is. Either way, some bulbs will work better than others. So my suggestion is, start with a daylight bulb, or a bulb with the highest color temperature you can find.
Kate