Now that's some interesting photos...it isn't down to wear from contact with the guide horns, that's for sure. The 2nd pic on the top is of a Befehls in early-war 2-tone camo (which included the wheels on smaller tanks like this one) so I think some of the variation you're seeing is due to that plus the possibility of dried mud/dirt making it appear lighter. The contrast in all the other three photos is much stronger, so some other factor is in play there. The last one looks like they are white-washed possibly since you can see what looks like snow or white-wash on the top fender?
Im not sure , maybe it could be something as simple as changed out wheels . They seem to be mostly the four wheels where the mechanism is , also second pic down has what looks like shiney new parts added to the mechanism ( forgive me I do not know what that part is called ).... the train lookin part
Definitely changes things to see the whole pics! The ones with the triangular-shaped antennas are most likely still sporting the pre-1938 three-tone camo as many units had not still converted over to the two-tone camo at war's outbreak, but that's just a guess without knowing when/where the photos were taken. The other two photos without the large antennas look to be from the Poland campaign since they haven't been up-armored or upgraded in any way and one has the tell-tale large white crosses, so they too could still be sporting elements of the old camo but hard to say for sure. One thing I love about German stuff, look at enough photos and you'll soon discover that there's exceptions to every rule out there that pop up all the time!