Aperture Tag: The Paint Gun Testing Initiative is a 2014 puzzle-platform game developed and published by the Aperture Tag Team for Windows and OS X. Unlike the official Portal series, the gameplay revolves around a paint gun rather than a portal gun, that fires two kinds of gel, one of which gives the player a jump boost and the other a speed boost. It also introduces other new mechanisms such as the paint fizzler and Pneumatic Diversity Vents that transport the player. The game takes place after Portal 2 and the player is guided by a personality core Nigel to complete puzzles in the Aperture Science Enrichment Center.

The game also features new characters, voice acting, along with a co-op mode that includes a level editor. The game's development began in March 2013, passed through the Steam Greenlight system in February 2014, and was launched on Steam on July 15, 2014. Aperture Tag was praised for the concept of the paint gun, but the writing and voice acting was criticized. Upon release, the Portal community complained about the mod being a paid product; Portal 2 is also required to play Aperture Tag.

Like the Portal series, Aperture Tag: The Paint Gun Testing Initiative is a puzzle-platform game in which the player must solve puzzles in "test chambers". While solving the puzzles, the player is provided with a paint gun that fires two kinds of gel, repulsion and propulsion, instead of a portal gun like in the Portal series. The two gels were featured in Portal 2; repulsion, colored blue, gives a jump boost and can be used as a bouncing pad, while propulsion, colored orange, provides a speed boost to the player. The repulsion gel can also be used to disable turrets. Unlike the stationary locations of gel in Portal 2, however, the paint gun allows the player to apply the gels to most surfaces.

The campaign consists of 27 test chambers, including re-used chambers from Portal 2. At the start of the game, the player must solve puzzles without a paint gun. Once the paint gun is obtained, the player can only use the repulsion gel; the propulsion gel is unlocked later in the game when the difficulty of puzzles increases unsteadily. By the end of the game, test chambers are themed with different biomes, including forest and ocean.

The game also features timed puzzles and toxic floors that kill the player upon collision. Throughout the game, the player encounters additional mechanisms such as buttons, spikes, and portals that cannot be placed by the player. An addition to Aperture Tag also includes the "paint fizzler", which disables and reactivates one or both gels on the player's paint gun when the player comes into contact with the feature. The game also takes advantage of transparent Pneumatic Diversity Vents, a cut part of Portal 2, that move the player from level to level instead of elevators that are present in Portal 2. The game does not feature the cinematic physics of Portal 2.

Aperture Tag can be completed in approximately 3 hours. The game also features a modified level editor, allowing users to create their own maps, which can be utilized in the single-player or cooperative modes.

Aperture Tag takes place in the Enrichment Center, the main development and testing facility of Aperture Science, following the events of the Portal 2 cooperative campaign. The Aperture Tag test subject appears near the end of the cooperative campaign. After waking up in a stasis chamber, the test subject is introduced to basic controls of the game as well as a personality core Nigel, who was designed by GLaDOS. Nigel then guides them to the paint gun while defeating turrets.

Once the test subject has acquired the paint gun, they must complete sixteen test chambers. Initially, the repulsion gel is only available, but the propulsion gel is unlocked after the seventh chamber. After completing the sixteenth chamber, the test subject told that they are supposed to enter the Aperture Laboratories Stability Stable Energy Reactor (ALSSER), but Nigel informs them that ALSSER must be deactivated. After fleeing the reactor, the test subject finds themselves in a set of chambers that are styled after different biomes. At the last chamber, the player can choose one of two endings. In the first ending, the test subject fights against turrets, and Nigel separates from the test subject, who subsequently burns in a fire pit. In the alternate ending, which is unlocked by closing the fire pit with a button, Nigel, as ordered by GLaDOS, allows the test subject to return to surface, which is later revealed to be a simulation.