I am not a Gimp user, but perhaps look into how you could copy the alpha channel (or transparency layer/mask, or whatever it might be called in Gimp) to a new image. The idea/rough process i am thinking of is this: Create a new image of the same size as your original png, and fill it entirely with white. Then, copy the alpha channel (transparency layer/mask) from the original png to the new file.

Still not a single action, but this would have the advantage of not requiring to fiddle around with color mixers or some color-related sliders, and it would give you full confidence and certainty of what the color of the new image actually will be (down to the actual and concrete RGB/HSL/HSV/... color value), simply because you directly and explicitely pick the concrete color the (new) image should be...

If you must do this often, and you really must have a single action, you might wanto to think about whether it would be worth it for you to invest time into turning this process into a Gimp macro or Scheme script.

As often with these things, the (official) documentation is your best friend.