Right now I'd say definitely colour. Good colours have always been the most difficult for me, and this last chapter I made a conscious effort to not just use obvious or easy colours in every scene -- I tend to default to "golden light, purple shadows" to make things look nice but there are like... ten billion other possible colour schemes. I'm trying to break out of being afraid of non-muted colours and figure out how to make stronger, brighter colours harmonise.
A friend who is very good at colour has been critiquing my in-progress stuff and helping me figure out how I can approach colour better, and I've also been (very slowly) giving myself a few chances outside the comic to practice putting colours down without a lot of overlays and multiply layers to try to force myself to think about colour more! It's all very challenging and I don't feel like I have it yet, but I've definitely hit some points I'm really proud of!