That's actually very hard with NetLogo, because the angle in an oval keeps changing.
NetLogo isn't a graphic design tool, like Photoshop, so some things that are easy to do in graphic design are nearly impossible to do with NetLogo. However, on the other hand, you can do some things with NetLogo that would be impossible in Photoshop as well.
Instead of making an oval, you might try making a sequence of overlapping circles of the same color. Sort of an "elongated circle"?