These are the parking positions. With the gray plane, you can also spawn your plane in the Cold & Dark state. With the red plane, it is not possible because this parking position is too small for your plane.
Yes, red parking positions are too small. Select a smaller aircraft if you want to start there.
With Aerofly FS 4, you can put your plane on the red mark and set it at ground as altitude.
Quit the location menu and go back in: you’ll be able to set cold & dark for example.
It’s a bypass of a bug, because the red mark means your plane is too large for this place!