rust colored? That I do not know about, but if the specks are silver or blue hued, that could be the beginning of cataracts. Perhaps Sandie might know the significance of rust colored specks. Sandie, you about???
Midnight had the same thing happen a few years ago. He has pale gold eyes and a rust-colored speck appeared in one iris. He doesn't appear to have any trouble seeing. He's 10 yrs old, so this happened about when he was 7 or 8.
I am by no means an expert on eye problems. The only thing I personally know of that can cause rust or brown spots on the eyes is Iris Melanoma. There's a few other terms for it and it can be benign, or Malignant. The only way to tell for sure is to have a veterinary ophthalmologist take a look at it. Most Veterinarians in general practice aren't very familiar with opthalmology. They are accustomed to treating some of the "familiar" eye disorders.