So Harper seems to be getting darker and darker. Could she be molting to a darker color?
Also Harper has a mustache! I just thought this little marking on her muzzle was so cute and weird looking.
Not necessarily. While it is possible to have different shades for different moults, sometimes fur colour can be darker while initially growing through and then the eventual coat is a lighter shade. My Mattie is just about finishing his moult and his new coat was coming through dark ginger, but now it's back to normal shade.
i could swear Charlie is changing colors, his floppy ears were a lighter brown at 6 weeks and now at 9 weeks, they seems to be more of a chocolate brown, his chest lightened up to more of a beige.
and his feet have gotten darker
Your rabbit is a chinchilla or light gray in color. This color like other agouti colors have bands of varying color. When agouti hair grows in you see the surface color first which is why it looks so much darker because you are seeing the pearl ticked with black first.
Chinchilla Color Description
Surface color is to be pearl white ticked with jet black. There is a black band on top of a pearl white intermediary band over a dark slate-blue undercolor. Chest is pearl white over a dark slate-blue undercolor. Eyes are brown.
For bunbunmommy your rabbit looks to be sable point in color and as these guys get older the points get darker. Temperature change as they get older can also change the shade of points