I am not aware of feeding parrots anything to keep the color but i do know there is something they give Red Factor Canaries to keep the red color.
I found this bit on canaries.
Red Factor Canaries
The Red Factor Canary (Serinus canarius) is an example of a color-bred canary, or a canary that is bred and prized for its color, rather than its song. These birds' body-types appear to be just like the other canaries, but with one special trait - the owner can influence the color of their bird. You may have heard that Flamingoes are white unless they are fed brine shrimp or other pigmented foods - this is true, and the same holds for the Red Factor Canary.
When the Red Factor Canary is hatched it is a pale peach or orange. It owes this original color to the Red Siskin, which was introduced to the canary line in the late 1920s. Most of the Red Factors, however, are color-fed, meaning that the owner feeds a special diet to create a bird that is a deep orange or red, much like those Flamingoes. You can recognize a Red Factor canary by this color.
All Canary males will sing, but the Red Factor is not known and bred for its singing - you may want to invest in a song canary if you want your house filled with beautiful song. Even though this canary is not "formally" trained to sing, it does have a pleasant song, and is not a noisy bird, like many companion birds can be. Canaries are quite happy in pairs, and you may want to consider a male and a female you make your selection. You can try your hand at breeding if you have a very compatible pair!
Canaries are gentle birds, and will not bite when you handle them. However, unlike most companion birds, they will not enjoy your close contact - this bird is best for the person who wants to add a bit of singing and beauty to their home. Even though your canary will not want you to hold him, he will recognize you as his owner, and may become quite fond of your company.
Your Red Factor Canary, unlike other canaries, will need to be color-fed in order to achieve the deep red or orange pigment that is specific to this type. You can find specially formulated color-food, or you can try your hand at creating a natural color-diet on your own: carrots, paprika, cherries, cayenne pepper, beets, yams, and any other orange and red natural food will help to change your bird's color. This natural way of color-feeding is purportedly better for the canary's overall health. Color-feeding should begin around molting time, when the canary is producing new feathers - the color will not appear in feathers that are already on the bird's body. If cared-for properly, Red Factor Canaries are reported to live for more than 10-12 years.