This keeps happening. I am using blender to render an airplane in X-plane. I keep having this problem where parts of the model turn funny colors after saving and opening it again. The parts that turn color seem to be random, and after they change I can’t seem to be able to do anything with them.

they look like the gray section.
I have had this happen a few times already. Last time it was worse and I had to rebuild the entire tip. (yes I know the tip is not straight, and how to fix it. ((YAY! thank goodness I know how to fix it not being straight at least))

That grey area indicates the active face, or the last selected face. It is not an error, it is a feature.