Change Color of Object Based on Shape Covering It

Hi. I'm new to unity and i don't have the slightest idea on how to do what i want. I was hoping i could get input so i can know where to start.

Let say i have three objects. One is shaped like a circle. The other is shaped like half a circle. And the last one is shaped like a thin rectangle

A - circle

B - half circle

C - thin rectangle

1. I use A as my base. I put B on top of it covering half of A. Then i put C on top of both of them.

2. I change the visible color of A (considering the shapes on top of it)

3. I remove B and C and A's color should look like there are purple slices.

Does anyone here know how this could be done in unity? Is there even a way to do this? Thanks.