DJI voids warranty for use of non-OEM parts (but only if they can prove you used them).
You'd put the original cover back on if you sent it in for warranty work. The screws aren't "tamper-proof" types that would give it away. So DJI wouldn't know that you temporarily swapped out the top cover..
But if you are still concerned about warranty just get a skin.
Once it's in the air they are all the same color from the ground anyway. Lol