Got your game face on

Is "got your game face on" considered an idiom? My son's class has to think of an idiom to be for Halloween. He was going to wear a game face (colts) as his idiom

It is indeed an idiom. The original, and I hope correct, meaning has been the subject of discussion on this site. Type "game face" into the search box on the previous page. However, the expression has entered a second phase, in which people assign a meaning to it as suits their needs, which are sometimes commercial. Your son can wear a Colts uniform or helmet to indicate the "game" part of game face, but he had better also adopt a facial expression, and perhaps makeup, indicative of the confident and determined attitude a Colts player must have in order to put fear, or at least respect, into his opponents.
SS

I think he might get a better grade by painting his face like a chessboard or something along those lines. I wonder if he's just trying to find a way around the assignment (i.e. to wear a "normal" costume) by going as a football player/fan.

Game face -- 1980s. U.S. black. One's public face. Cassell's Dictionary of Slang by Jonathon Green (Wellington House, London, 1998).

I think the above is in agreement with the archives. A face that is devoid of emotion, intimidating. Not letting your opponent see you sweat. On the other hand, I had a class with a young woman who was always smiling. I remarked on it. She said, "It's my game face." Same thing. People couldn't tell what she was really thinking and feeling.