Plumage color in canaries is due to coloring pigments carried to the feather by the blood. The degree that the color is expressed is affected by the kind of food that is consumed. Birds in the wild take food from various sources including berries and pollen that result in rich colors. A captive cardinal's color will turn grayish pink whereas in the wild it's color is vibrant red. Beta carotene enhances both yellow and orange and is found in many types of greens and abundantly in carrots. Spirulina being marketed as a booster for the immune system is said to have 20 times the amount of beta carotene as carrots.
Yellow color is due to yellow lipochrome, which is of an oily, fatty nature. Yellow appears in two distinct forms; a richer deeper form is called "yellow" and a paler for call "buff". In the buff the feather is thicker and the color is not carried to the edge of the feather, consequently the edge of the feather is white and the overall effect may be a bird with a frosty surface. The darker yellow is commonly referred to as "hard feathered" with the color extending to the edge of the feather. Many breeders claim that we have bred the yellow out of American Singers and Cliff Williams didn't identify his birds as being yellow, as darker colors were noted with the letter "R" denoting rich. For instance a clear bird was classified as BC-R meaning buff clear-rich.
American singers having a hard feather produce, darker yellows, and vibrant greens. Mating hard feather birds together on a regular basis is not a wise policy as the feather texture will become too thin. Double buffing, the practice of pairing two buffs together in other breeds such as Glosters and Norwich is reputed to be the cause of feather lumps. To avoid the excess of feather caused by: "double buffing" or the scarcity of feather caused by hard feather to hard feather matings it is generally a recommended practice to mate hard feather to buff.
Hard feather yellows intermated can produce buffs, and buffs intermated can produce hard feather yellows. A single yellow can produce as deep and rich color as two yellows mated together. Whether hard feather birds can be produced is based on their genetic background. If your desire is to darken your yellow plumage or change olive green to grass green, the solution is to introduce a hard feathered bird.