I gave it some time to see if someone with better expertise would show up to give their opinions, but I guess you're stuck with me! xD
So, about adding more colors... I think it's fine. If you stick to whichever small amount of colors you think is reasonable... I mean, ultimately it's your call, right? It does feel weird to me, though (and that's just my personal preference) when shading in a pixel art image is too smooth. So, IMHO, and to my eyes, your newer image looks better.
The mouth is hard, isn't it? It gave me a lot of trouble too. You might want to try to do straight copies your references to try and get a feel for how to represent it and her other features in this piece (though not necessarily in pixel art).
Her eyes look thinner to me, so for that, your older version, from these last two, is a bit more accurate.
As for the ponytail, if you decide to keep it in the front and over her shoulder, with your extra colors you could probably smooth it out like the strands in front of her face, even using some pink; I think it would catch some of that light.
Some people don't like "too much AA" in pixel art, but I mean, the colors are there, why not use them? Unless you set for yourself other restrictions, like only having a certain number of colors per "cell" (8x8/16x16 area)...