Highlight color change borders?

Question

C++ / C#

What algorithm can be applied to this problem?
There is an image at the input, you need to color the borders of the color change. Image without gradients.

Answer 1

https://docs.opencv.org/3.1.0/da/d22/tutorial_py_c...

Answer 2

You can do this
UPD:
I didn’t write the code myself, but the first edge detector I found online gave this. So I think it suits you.