I started blender couples months ago, and I wanted a character with a torso similar to Mario (round). I imported my character on Roblox studio but then I saw that the lower part is a little bit darker than the upper part of the torso. I recreate the torso thinking that I accidentally did something wrong, but same issue. I thought that the problem came from roblox and I did a little bit of research but I found that the problem is about 'recalculating outside normals', but unfortunately, nothing change.
But then I saw that separating a cube is different than separate a sphere. The cube make it less obvious that its two part, but each times I separate a sphere but smaller part becomes more darker.
Also, I have no lighting, when I paint my mesh with vertex paint the same issue occurs (for example, I paint red and the lower part is a darker red), I also tried with material but same issue.