Boomcorals, don't you think a maricultured acro needs more than weeks for it to change color? Ime they all change at some point at least a little. Some will take a number of months to change hues.
Cody, yellows that are wild or maricultured are hard to keep sometimes. A lot of the corals coming from natural sunlight look yellow but will turn green without proper light and nutrient cycle. There are certain species notorious for looking yellow when arriving and staying green after aclimated to a tank. If your wanting true yellows there are a number of tank raised corals that are great. But if they are not a popular named piece make sure you see them in person or know that others have been able to keep them yellow, pics are sometimes misleading as well as labels. The oldie but goodie acros that are yellow are good starters to learn how to keep the color vibrant before trying wild pieces.