^^^ what blakey said!! heres a bit more detail though;

basically, with how uploading paintings to the gallery works, every painting is sorta "copy-locked"; you can copy any drawing from someone and edit it, but it will not let you upload it due to it being from another arist, to prevent art thieves.

"copy-locking" is tied to accounts, so the main way people get around the copy-locking for collabs is to make a shared account, with a password that the collabers both know so they can copy each other's drawings!! this is the only way to do collabs right now, but the colors devs said they had plans to make it a native feature in the future to prevent issues that can crop up from account sharing