Without a known reference colour in the picture, it can be difficult to be sure - in general, older is ortho- and younger is pan-, but there was a lot of overlap time. Yellow is the most awkward colour - it can be dark or light. As you can see from the two pictures, you have little chance of being sure, unless there's something in the shot that you can be confident of. You could have two B&W pictures of the same yellow stripe and see one as dark and one as light on the final print, yet the engine would look the same grey in each picture.