No, it's a myth (and a popular and much-repeated one at that!).
My 4 month albinos have black toe tips, thier wildtype siblings have white. Nowhere near maturity. I think at best you could describe the toe tip colour changes as the transition from juvenile to sub-adult, or from child to adolescent.
I have a five month old axolotl, and she’s not even close to six inches! I think she might have growing issues. Shes only 1/4 to 1/2 of an inch longer than she was at three weeks. And I got her from a good breeder!
i highly recommend “buyanaxolotl.com”- ive purchased from them and received a beautiful animal for relatively cheap, in great condition, and excellent shipping precautions. the breeders are a couple living in georgia (i believe, don’t quite remember) and they’re fantastic. sometimes their website contact page doesn’t work, so i’d probably try just emailing them. good luck and happy hunting!