Bottomline: your pens will not be affected by indoor lighting, but you don't want to leave them out in the sun or store them by a window. As JLS1 pointed out, exposure to UV does affect urushi lacquer and will also lead to changes in color over time.
I thought that the gradual changing of urushi color--specifically, like the green in heki-tamenuri becoming more noticeable over time--was both natural and desirable. Is that not so?
Eo, you raise a good point. I've assumed that it is oxidation that is involved in the transformation of urushi on aging, specifically the change of the slight natural color of the top layer to coloress (allowing the underlying color to become more pronounced). It may be a photooxidation process that is involved though.