Aperture Tag
Aperture Tag: The Paint Gun Testing Initiative is a mod for Portal 2 made by Odd Time Studios which focuses on the "paint-gun," an alternative portal gun which dispenses gels instead of portals.
Linux support
Although the store page claims to have Linux support, it will not open from a fresh install. It is, however, possible to get the game to launch, by copying Portal 2's /bin folder into the Aperture Tag installation folder, or launching it using Portal 2's launch script. More information on this method can be found at this Steam Discussion thread. However, due to it using Portal 2's binaries, SAR will erroneously think the game is Portal 2 rather than Aperture Tag, and is unlikely to be considered legal for speedruns. It is for this reason that SAR's support for Aperture Tag is listed as supported with Proton, rather than full support.