Can Krita erase just a single color?

You could do that with the "Select similar color tool" and then clear.

Yes you can do it also with a brush too. ( As far as I understood 'Erase' as 'With an eraser' )
Well, the feature exist, and I tested it long time ago. I tested today with the developpement branch, it seams the feature is actually broken. I will report it.

For the feature , it was this way :

1) preset editor
2) Engine 'Filter'
3) In the filter option menu
4) Choose 'Color to alpha'
5) Select color and threshold ( well, color selection is not really precise too in this menu , I would rather like Krita to use actual foreground color )

Gives feedback if you use a specific version and you got it to work.

thanks all