It is two photos that have been blended to look like one.
Notice in the color side the back-ground buildings are rather ordinary but the buildings in the B/W are war-torn.
It is a statement photo, the brightly colorised new soldiers listening to the war-weary older vets.
The men on the right have their shadow falling on a almost flat surface which is perpendicular to the way they are standing. While those on the left as standing of debris or something else and in front of a building. So of course the shadows would be different.
Insofar as the shadows, the shadows are aligned with the direction of the light. the half-finished colorization is simply still incomplete, obviously meaning the person did not finish what he/she started doing and we'll never know why since that person is probably long, long dead... unless the colorization is a modern colorization done through computer digital enhancing techniques.
Quite likely, in fact almost certainly, the part on the right is a recent posed photo. There was no practical form of colour photography which could be used "in the field" in WW1, it does not seem to be a "coloured up" part of a b/w photo. You can also see that the road surface varies between the left and the right side.