I wouldn't rasterize a single-colour red ellipse on white any differently from a single-colour green ellipse on white. Those are still contextless shapes!
But, I would rasterise a bright green plastic pipe (the opening of which is an ellipse) sticking out of grey concrete that's meant to be the focal point differently from how I'd rasterise a grey hole in that concrete (also an ellipse). That's context. And the reasons I'd do that are
1. The green pipe is a focus, and so I would probably make it a little thicker so that it's clear that it's the focus.
2. Light affects plastic and concrete differently, and the two objects have different types of edges. This effects how much of an impression of light/shadow I want to give and how smooth/soft I'd make the edges.
3. The two objects have different textures, and possibly different kinds of wear and tear. I would break up their outlines to reflect this. Subtle chips in the material are very easy to convey by choosing to rasterise one part of a shape differently from another, since the overall shape is still an ellipse, but now with subtle irregularities.
See what I mean by context now? Actual artwork. Scenes, tilesets, etc. where ellipses are used to depict objects or create an impression rather than drawn for their own sake.