How to create opacity mask from colored bitmap?

Question

For the image (bitmap) below,

if I do:

1. Create a spite with the image in unreal engine.

2. Create a opacity mask on the spite.

3. Use this image, choose a specific color, such as blue, make its corresponding region the only opacity region on the mask.

My question is about the implementation of part 2 and part 3 in Unreal engine, they are quite confusing.

If anyone know how to implement it in unity, it would also be helpful.

Thanks a lot.