Do you have any pictures?
My betta started out with blue and red color and as his fins grew as he got older, they came in clear. They darkened up after a couple months. Is the clear only on new fin growth? Or was he always clear to begin with? At 4 months, from what I've seen, most bettas would get the majority of their color, if they don't have the marble gene.
How do you know it was a mustard gas betta? I don't think petsmart could accurately say what kind of coloring it would have if you bought the baby that young. They usually just label it as "baby boy" or "baby halfmooon" or something.
They can be a drastic change from kid to adult (see my pictures, I also bought a baby boy from petco and you can see that the new growth was clear before it darkened up to blue. He also lost all of his red in his fins.) but they should have color when you buy them as a baby, especially if its a mustard gas, which should have pretty dark blue/green or black coloring for the body. If the body is still light, my guess is that your betta isn't a mustard gas.