We are adding language icons for Fortran in VS Code i.e. the icon that shows up next to the file name, currently the plan is to use the Fortran-lang logo for both Free and Fixed forms, in both light and dark theme.
If anyone has a better idea and is interested in contributing some artwork this is the PR: feat: add language icons by gnikit · Pull Request #622 · fortran-lang/vscode-fortran-support · GitHub
Using the Fortran-lang logo for all source source seems like a good choice. Personally, I would use it for both free and fixed form source in the Fortran-lang violet. If you want to make a visual distinction, best not use the cardboard brown for fixed form source, we have several other colors which go nicely with the Fortran-lang violet (see Choosing a license for logo and other artistic assets · Issue #342 · fortran-lang/fortran-lang.org · GitHub for an example).
For fpm.toml you could use the fpm logo, see The fpm logo — Fortran Package Manager.
Is there anything else which requires an icon? Depending on what you are aiming for I can give it a try.