Is it possible to copy red channel to alpha channel in gimp?
This does the job well:
Colors->Components->Decompose
- Set Color Model to RGBA
- Uncheck "Decompose to layers" - my personal preference
- OK
Colors->Components->Recompose
Swap the RED and ALPHA channels
It seems that "Colors -> Color To Alpha" should do the trick if you set the color to ff0000.