Couple of 'old-school' ideas that should work: 1) bleach solution. Probably 50/50 bleach and warm water solution should get a lot of the grime off and restore the white. 2) Go to auto parts store and look for Wesley's Tire White. Comes in a spray pump plastic bottle. We all used to use this stuff on car tires that had white-walls and raised white letters. Spray on, wait a few minutes, scrub off with a nylon bristle brush. Should whiten leather right up for you.
Last tip: When you get them back to white, polish them with transparent shoe polish. This will put a hard protective finish on the shoes that should resist grass and dirt stains. Clean and polish them a couple times each season to keep them white longer.
Good luck and let us know how clean and new looking you can get those shoes.