I spent a while looking for this kind of set up when I first started playing with UMG.
Here’s a quick dirty set up that works well. It’s late so that’s all I have the energy to make.
You have to set the Text to Is Variable.
Make sure to toggle between context sensitive if you can’t find the widget you’re after!
EDIT: I should really say what that does - Every tick it checks if the button is hovered, if it is, it makes the text black else it is white.