From your photos (first two posters), it looks like your rabbits are Californians? Or at least have pointed white coloring of some type.
The gray you're seeing is commonly referred to as "smut" (color spoiling a solid coat) and is not desirable as far as breed standards go, but it's not harmful or indicative of any health problems.
It could be seasonal, since pointed rabbits often get darker in cooler weather and lighter in the summer, or it could be an age-related genetic factor. Either way, it's completely normal! You'll notice the same thing on colors like sable point where the color pattern may appear kind of dirty (rather than clean solid cream/white), or on colors like orange where you'll notice varying shades of color, from vibrant to smutty.