I think it is definitely more than the lights. Fluorescents do affect the way colors look next to the skin (as it also wrongly displays skin color) so it's best to know how a color already looks on you if you're stuck withcrap lighting in a shop (or bring the item into natural light outside the dressing room and hold it up next to yoru face in front of another mirro if there is one).

But I have noticed that the mirrors themselves distort at times; making one look taller and thinner or shorter and fatter. They may just be substandard mirros, or hung this way intentionally. The fancier the shop, the more I tend to notice the mirrors are quite flattering; which of course leads me to hope these are "good" mirrors and this is the way I really look! :) Of course, most of the time I tend to buy my clothes at thrift shops which usually don't even have dressing rooms...

I like dressing rooms which show a rear view as well as a front one; it's weird to get this view as one does not usually have the ability to see in this way. Sobering at times, but nice to break down one's illusions, too!