These are very cute! I think they would benefit a lot from AA and from avoiding something called "banding".
Banding in this case is occurring on the swirl shapes in the liquid itself. The idea is not to have lines thicker than 1px and when adding a slightly lighter or darker line next to one, not to have them the same length as the first one. It's really hard to explain XD But adding AA (anti-aliasing) would help to explain a better way of doing the swirls in the middle.
I don't have any tutorials set up yet to explain all this well, but I circled some examples of where banding is happening on here; [link]
And this is how I would modify it to reduce the banding problem as well as introduce more AA (you have some in there now but I think with round things it smooths them out to have much more): [link] (before and after zoom) [link] (before and after actual size)
I've added three extra colours, added a lot of AA and removed all the banding and lines thicker than 1px for the line art : ) Sorry if it bothers you to show you this way, I just don't know how else to explain at the moment XP
Wow that's incredibly helpful of you - I'm new to 'pixelling' so it's really wonderful that you would take the time to show/explain me this. I'm definitely going to rework them and I'm really seeing the difference, the swirls look much smoother. Thanks again <3