Before you go to the dying stage, I'd use some saddle soap and a toothbrush and scrub the entire panel. When I did that on my hasp bag, it made lighter areas. Then, later, with leather CPR and blackrock leather 'n rich, it evened out.
For color fix, here's what I would try. Some leather dye (I use Kiwi brand) mixed with leather CPR. Rub just a little of it on the lighter area and see if it evens out the color. Let it dry. Maybe more than one application. Now, if your bag is putty color, you'll have to find a color that's close.
If you want to dye the whole bag, I think that's doable and in some ways easier. Again, I use kiwi brand leather dye in a little bottle with a sponge top applicator. I've had good results, mostly black bags, but also brown.