I can’t give you a super scientific explanation of the exact chemical reactions that are going on, but what you are seeing is a natural tarnish and corrosion resulting when any “clad” and copper coins are buried in the ground for awhile and begin to react with the minerals, chemicals, and rain water. Modern U.S. nickels, dimes, and quarters (1965 and later) have enough nickel content that they always come out of the ground with some varying degree of grey, brown, or even reddish or orange staining or tarnish even after the dirt is washed off. The process happens pretty quickly. I once buried a shiny new modern nickel in my backyard as part of a “test garden” - after a few weeks, I decided to remove the nickel, and it had already started to permanently tarnish!
Pennies from 1982 and earlier are mostly copper, and can often clean up pretty nicely with just water, although the older ones gain a nice green patina or other kinds of natural copper corrosion depending on the soil. Pennies after 1982 are mostly zinc and literally get eaten away by corrosion in the ground in very short order. I’ve dug 2015 “Zincolns” that are already destroyed by corrosion.
Silver coins (mostly dimes and quarters prior to 1965) often come out of the ground with little to no real corrosion, depending on soil - a sight I never tire of seeing!
Anyway, I would guess that different lawn treatments or other slight chemical variations in the soil might be the reason for the different colors on the coins despite the soil looking similar.