It is common for a Poodle's coat to dull, become brassy or yellow as the dog matures. Sunlight, air pollution and constant bombardment of minuscule debris are at work here, in addition to the natural aging process cycling out a duller color than a Poodle once had.
At all times, each hair strand is in 1 of the 3 phases: Growth, rest or release. Therefore, it is a gradual process... some hairs grow in while others are resting and still others are releasing (shedding off).
Owners may not notice a difference in the richness, depth and shine of the coat until the process has been going on for a while and enough hairs have grown in faded or dulled to make a drastic difference.
The key to fixing and maintaining a beautiful color on a Poodle is to use a color enhancing shampoo. There are some great ones manufactured specifically for canines. In addition, while in general one does not want to use a human product on a puppy or dog, as long as the PH balance is correct, it most certainly can be used with excellent results.
If you want your Poodle coat color to be richer and purer, but are not sure which product to use, you may wish to look to 'Grooming - Shampoo - Color' in the Poodle Specialty Shoppe