There are several ways to find the circumference of a circle. It depends on what information you have. Some information helps, some doesn't.
You can calculate the circumference if you know the circle's radius, or its equation, or its diameter, or the length of a piece of it and the central angle formed by the piece.
On the other hand, if you know the circle's color, or its eccentricity, those won't help at all.
You use whatever you've got.
