I always assumed that Mosskit was oddly and probably colored wrong due to a mistake but the more I think of it, the more this makes sense.

Mosskit didn't get the same blue color as her mother, or brown color of Oakheart.

She got her colors from Snowfur and Moonflower.

Snowfur gave her niece the white fur, and Moonflower (a silver gray tabby) gave Mosskit her gray spots.