I understand that you are concerned that two your pup's toenails have changed from black to an opaque white color.
The good news is that she is not limping and the toes themselves look normal.
Dogs can have vitiligo, which is a nonharmful loss of pigment. We can see patches of white fur on black dogs or loss of color to the nose, eyelids or toenails. This is thought to be an autoimmune disease (body attacks its own pigment producing cells) and is more of a cosmetic issue, as the tissues themselves are normal, other than a change in color.
But if the nails are soft or uncomfortable (and they may be because she is licking them more) this may signify a bacterial or fungal infection. Those can be the primary problem, or they can develop secondary to her licking and increasing moisture which leads to tissue breakdown and allows a secondary infection to occur.
There are other more serious diseases that can affect the nails such as vasculitis (blood vessel inflammation) and other autoimmune diseases like lupus or pemphigus as well as endocrine diseases like hypothyroidism which cause of nail abnormalities.
At this point if her nails are the same hardness as her other nails and her toes look fine I would try to get her to stop licking (an e-collar also known as a lampshade collar or cone of shame may be necessary) and see if her nail color returns.
But if her nails seem soft or overly brittle, her nailbeds are red or have any sort of discharge then a recheck with her veterinarian is best.
Please let me know if you have any further questions.