Is there any way to calculate a proportion of a certain area size to the entire area size in an image?

Hi

I'm doing an image analysis and trying to calculate a ratio of a certain part to the whole in terms of their sizes.

A sample image as below:

I'd like to esimate how much the purple part at the center accounts for of the whole (whitish circular) area.

Is there any way I can do that in Matlab?

Thanks a lot in advance :)