The question has already been asked, but the accepted answer is really not what I'm looking for.
I want something that measures the alpha value of every pixel, and turns that into a given colour. So, for example, if I have a yellow pixel with 50 % transparency, and I "alpha-to-colour" it to red, then it becomes orange and fully opaque. A plain yellow pixel with no transparency would not be touched.
In other words, it's really like a perfect "undo" of colour to alpha.
Up to now, I have no clue how most layer modes work, but if there is a way of using a layer which has been filled with a plain colour to do what I'm looking for, that could be an option. I have no clue how I should ask this to a search engine in less that thirty words, so I decided I'd ask humans for a change.