Do you mean an ALL white scopas? I have never seen those... although I've owned "tricolor" tangs also... the mutant variant of Zebrasoma scopas, and those will change in color too... interestingly the light colored markings morph back but the dark stayed for me... here is a scopas specimen of mine...
The tail was originally all white... in this photo you could see it waning...
And further on here the white is almost completely gone... the black on the back stayed the same... it's tough to tell from the pics, but the color on the back is just like the black/blue you see in Zebrasoma rostratum...
There is a regional morph of Zebrasoma scopas that is all yellow that I have... this one holds its color...
Yet many of morphs that are lightly colored aberrants/ piebalds/ xanthics will morph back over time... this has amazed many ichthyologist friends of mine... but it is true... here are some of my other fish through different families that I've noted will morph back... something to keep in mind before you spend four figures on a fish!
A Hawaiian flame wrasse Cirrhilabrus jordani right after collection...
... and a few months later...
It eventually changed back to normal...
A blue angel Holacanthus bermudensis when I got it as a fully xanthic juvie...
After a few months morphing back...
And now older (and normal colored)...