I have dealt with game artists everyday for many years. My background is computer science, that is why I find it challenging to understand them well. Someone said to me once “you will never understand artists” laughing at my face.
Something I know, anyways. All the people, all the time, give their opinions to the artist. I like it, why blue? Mmm I don’t know. Honestly? I would put that on the right.
You have to be efficient in talking to them. The best way I know? Describe to them what you are seeing. Use your honest and sincere words and wait for their comment on your description.
If something does not convince you, ask them questions. You are a game designer, but the game design is something the whole team will do. Also them. Do not lose the opportunity to use their brains. Facilitate the act of game design and respect their ego. Which, in some cases, can be very high.