I was reminded recently about people taking line art someone has done and colouring them in. Personally, I don't have a problem with it (I think it's actually quite flattering someone would want to work with one of my pieces) but I've seen some artists get upset about it. Maybe they didn't ask the artist's permission or something... For the purpose of practice, I think it's ok - providing they don't claim to have created the entirety of the piece and preferably ask for permission in the first place. What I like about these is that in the hands of someone else, one interpretation transforms into another by how much the other artist changes or adds to it.
Here's one example from a very old sketch I've done
Here's the colour by He did ask for my permission and a good job he did on it too
What are your thoughts on someone colouring/editing works by others?