About the pictures - I think you should have some kind of pictures edit option on your phone even - look for the 'exposure' as adjusting 'brightness' will just apply a film to everything and you want your colours to remain vivid.
Regarding the battle damage - I think some of the problem might be in that it seems you changed the approach . On your Intercessors you were doing battle damage by making dark markings which you later underlined with the highlight colour. On the Librarian, some scratches do not have this darker colour, which makes the scratches look like they were on the same level as edge highlights. And this makes the armour look flat. On a cloe-up, I can see that some of the markings on the Librarian have this type of 2-colours battle damage marks, but some don't.
Layering is a basic painting technique where you change colours by applying them in layers very close to one another to achieve a transition effect.
Stippling is a technique where you apply the paint in a jabbing motion (so striking the surface with the tip of your brush). This creates dots of colour which can blend together. Again, this is a way of achieving ta transition between colours.