Coloring embryos by injecting dye into eggs before they hatch has been practiced for a number of years. It is done to identify the young of certain hatches or groups. And it makes it easier to observe movements of wild birds (especially water fowl) after they leave the nests.
The process of coloring chicks by injecting dye into the eggs also provides an opportunity to study early feather growth. Juvenile plumage will replace the colored down in about two weeks. As this happens, the dyed background amid new growing feathers provides a constantly changing pattern.
While it is possible to inject eggs from about the 10th to 19th days of incubation, the period from the 11th to 14th days appears to be ideal. Only one treatment is necessary if the injection is done at this time. When injections are made after the 14th day the color usually remains localized because the embryo occupies most of the egg; so it may be necessary to inject the egg in more than one place.
Harmless vegetable dyes, such as food coloring dyes sold in grocery stores, work very satisfactorily.
This does not harm the chicks in any way, and eventually as they mature their adult feathers push through and they develop normally with their standard coloured feathers.