The key color is changed.

You should use keys and doors of color cyan, magenta or yellow (CMY) when you want to carry a key across a section, including going through a loop back to the starting point. These three colors of keys would not drop upon death, and these three colors of doors would remain unlocked forever. Failure to do so may cause soft-lock if the player get the key and die (or suicide) before the nearest checkpoint after the corresponding door.

The same point applies to your level 12 (https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3446793002) , though the comment area of that map is closed somehow and I can't leave any comments there.

You'll find a yellow key in official level 3-b, plus a cyan and a yellow key in 3-c. We'll try to improve the appearance of these three colors of keys and doors, to distinguish them from the other three.