Hi Don,
Take a good look at the OLD feathers on your Buffs. This is the perfect time of year to do it. Those old feathers fade, and tend to develop a soft Wheaten pattern with the shaft being lighter too.....shafting. Buffs, genetically, are really Wheatens. The trick is to breed the the wheaten markings in the same shade of buff, with deep Buff shafts to the roots to lessen the wheaten effect, and make a solid gold bird.
The more carotine and oils are fed, the more intense the color on Buff. Canary breeders have known this for years..yup, bred those too... and routinely color feed their birds. I've seen them produce gorgeous red and golden birds with color feeding. Those birds on their next molt, if not color fed, will molt out into pale, washed out birds, much to the surprise of some new owners.
Feather quality...substance of feather, will also affect shafting, and can be greatly altered by diet. I'm sure you've bought a few birds in poor feather, molted them, while feeding them a better diet, and have seen them vastly improved in feather quality and color. I know I have. I've also seen it work in reverse when I have sold birds in the past who were correct in color, to owners who skimped on diet. Those birds end up as very washed out shadows of their former selves.
And my recipe is......Bugs, sunflower seeds, and eye of newt!