After you quit crossing in other things, and just breed them to each other, it will still take at least a few generations to get them breeding true. The more chicks you hatch, and the more selectively you cull, the fewer generations it will take. I would expect at least 3 to 5 generations in the...
It partly depends on what you already have.
It also depends on whether you want them to be a color that is recognized in other breeds of chickens, or whether you just want them to look alike other.
One possible path: look at what color they are when they hatch, pick the most common appearance...