I thought of it as

P: A or B

P: ~A

Conclusion: B

It is essentially saying either you're red or blue, you're not blue, therefore you're red.

I found this easier to comprehend, and it is also logically equivalent to what the lesson wrote (~A --> B)

But in my case, I guess it also accounts for

~B --> A

This could be an easier way to understand this/map it out.