If someone has a little, almost imperceptible, leftover nail polish, would it be considered a chatzitzah with regard to mikveh immersion? If so, why?
This source gives the general guidelines of what is considered chatzitza for mikvah:
According to Torah law, a barrier (chatzitzah) invalidates a woman's immersion in the mikveh when it meets two conditions:
(1) it covers the majority of her body;
(2) she minds its presence – that is, it is a substance that she considers foreign and plans ultimately to remove.
The rabbis decreed, however, that a substance meeting only one of these conditions (either it covers the majority of her body or she minds its presence) also invalidates the immersion.
Thus, in practice, a barrier of any size is considered a chatzitzah if it is destined to be removed. Even when the woman does not mind the barrier's presence while she is immersing, if she would normally remove it at some other time (e.g., a ring that she removes only when kneading dough) it is a chatzitzah. Furthermore, if most women would be particular about an item, then it is considered a chatzitzah even for an individual who does not mind its presence.
Bet Yosef Yoreh De'ah 198:17 delves into some detail explaining the S.A.'s general statement that for someone who's work deals with colors (such as a painter) and has color on her hands, this would not be a chatzitza. Part of his reasoning is that women working with colors are not particular about what the coloring appears like on their hands (i.e. - they are used to it.)
However, Bet Yosef states that this ruling was made only for women who work with colors. However, for other women, even if they personally are not bothered by left-over coloring (which would include nail polish, as this is "coloring" of part of the hands), this would be considered a chatzitza, b/c most women are particular about how it looks.
This is only one opinion, and as stated in the Nishmat article (1st link), it seems to be what the majority of women follow.
You may want to read this related question on the Nishmat site, as it may be related to your particular situation.