I cut my own bangs, and sometimes a few face framers. I leave the rest to the hairdresser. However, not too long ago, my hair was getting pretty long, and the weather was oppressively hot, and I just got aggravated and grabbed some scissors and snipped about 4 inches off the back -- of course, I then had to wear it in a ponytail when I left the house, until I had a chance to go get a real haircut, so no one would see my ragged ends. And...here's the clincher...even though I was wearing my hair down around the house, my husband never even NOTICED the 4 inches missing from my hair. So...it's entirely possible that GR might not have noticed a new haircut.
HOWEVER, saying "she MAY have changed her hair color" doesn't make any sense at all. Did she or didn't she? If she'd had it done at the salon a day or two before, I would think that one of her 3 kids or her husband would have known that her hair color had changed -- and what color it was. If she'd changed it the day she disappeared, the box would have been in the wastebasket, and once again, the new shade could have been announced.
And, if GR found hair clippings in the waste basket, the amount of hair in the wastebasket along with whether Jenn habitually trimmed her own bangs, could have let us know if it were just a little fringe trim, or if she'd cut off most of her locks.
Quite frankly, I would guess that any changes to her hair would have been minor -- from the photos, and from a more accurate description of her personality, it appears that she preferred to wear her hair long, and was not the type to make dramatic changes in hair or clothing.