Nipples: https://e621.net/tags?commit=Search&search[name_matches]=*_nipples&search[order]=count

Areola: https://e621.net/tags?commit=Search&search[name_matches]=*_areola&search[order]=count

e621 currently has tags for both nipples and areola. Is there a purpose to this? I'm not sure if there is a reason to tag both purple_nipples and purple_areola when the former should cover the latter. Similarly, I'm not sure if huge_nipples vs huge_areola is meaningful, the very first example image for huge_nipples, post #2742896, in fact has small nipples and a huge areola.

I am wondering if we should alias all of the *_areola tags to *_nipples.

This is like saying we should alias *_pupils to *_eyes.

Having X-coloured nipples does not mean also having X-coloured areola.
For example, post #23058 shows pink_areola and white_nipples.

As for the example thumbnails on huge_nipples & huge_areola, it seems to be a mistake done by a single user. They should be flipped in reality.

There are 61 pages of posts tagged huge_areola and not huge_nipples. That's a sizeable chunk relative to the number of posts tagged with either, and most don't look like mistags. The colour tags, on the other hand, seem to be very redundant; most of the images tagged with one either have the other, or clearly should.

For example, post #23058 shows pink_areola and white_nipples.

That looks like a mistag to me. Small pink areolae and pink nipples. The white bits are specular highlights.