Sprites on old systems often used black outlines for clarity, so that their bright contents didn't bleed into bright backgrounds on crts.
Plus, when you only have 3+transparency, black is the universal separator. Megaman had a black outline and bright interior so he worked on either light or dark backgrounds. Imagine cyan and sea blue megaman disappearing into the sky

However! You are working from a top down perspective. You do have the liberty to go outline free as long as your backgrounds aren't blue. But the cyan is going to be a pain on light grey etc. So you might as well use black for the purpose you stated to separate the body parts.
Here's something cool, thanks to:
https://piratehearts.itch.io/supercrt
Anyway, there's a reason people used black pretty universally. If you're interested in making retro sprites,
the process behind achieving the CRT effect is here. A big part of.... why my NES was so beloved as a kid and contained magic that it lost when I got a bit older and started emulating, is that the medium of viewing was actual magic by comparison. There's so much mystery hidden in the degradation! GBC is where NES-like graphics were developed for LCDs, so if you're developing for modern computers without the goal to distort, GBC may be better to learn from. I suspect the majority of people getting into making retro style graphics are also predicated against screen effects, which is confusing when you think about it. But this means even if you don't create for the intent to view on CRT, you have a lot of company.
...I'm gonna buy a CRT.
When I first made the sprite, I was pretty happy with the result, about an hour later I thought, "I think I messed up with that sprite. In hindsight, it's pretty bad."
That's excellent. Your brain went through enough change in such a short period to see the same thing completely differently. It means you're learning a lot
