When you cross two breeds to introduce a new color to a certain breed when does it go from being considered a "project" to being a purebred again?
I agree with Fred, but would like to add an additional thought or two.
In the science world, the term purebred can mean an animal is homozygous at a specific gene locus. You can have a bird that carries two recessive genes at a specific locus and the bird is purebred. In another bird, there can also be a pair of dominant genes at the locus and the bird is still purebred.For example, at the E locus, a bird can be purebred for extended black while another bird can be purebred for wild type at the E locus. If you cross the two previously mentioned birds then the offspring are hybrids or heterozygous.
The term purebred, as associated with chicken fanciers, indicates the bird will breed true (this is not true for genes that are incompletely dominant). If a person crosses a purebred rhode island red male with a purebred rhode island red female, the genetic makeup of the offspring should be the same as the parents (usually, but not always). If you cross a purebred female delaware with a purebred rhode island red rooster, then the offspring are hybrids: males will have a silver phenotype ( what the bird looks like) and females will have a red phenotype. The delaware and the rhode island red share some genes that are exactly alike. The columbian gene is a gene that both the rhode island red and delaware share. Even though the offspring are hybrids, they can be purebred for a certain gene and in this case they are purebred for the columbian gene.
If you mate a rooster and hen, and over 100 offspring are hatched from the crossing, and all the offspring are the same phenotype. The chance is very good that the rooster and hen are purebred. if you take the 100 chickens and put them in a big pen and let them randomly breed and over time the offspring are all the same phenotype; the original parents and all the offspring are purebred.