To create other colors, you would cross a Dominique with a chicken of the other color. Then take the crossed chicks and breed them to each other, or back to Dominique, or back to the other breed. Use various combinations of those patterns until you have chickens with the correct color, with all other traits being correct for Dominiques, and they breed true (breeding them together gives more of the same, not a mix of traits.) This is the basic way to create new colors in any breed of chicken.
Wyandottes have some traits in common with Dominiques (dual purpose, rose comb, yellow legs, brown eggs). If you make other colors of Dominiques, people may think they are Wyandottes instead.
The barring gene puts white bars over whatever other colors the chicken has. Some examples of barring over various colors would include Rhodebar, Delaware, Cream Legbar, Bielefelder.
Yes, Plymouth Rocks might be a good choice for introducing other colors. Wyandottes might also be a good choice.