Barring is dominant but it is MUCH easier to get it from a barred roo because he carries two copies of the gene. Females only carry a single copy. Using a solid coloured male with your barred girl, you could end up with 50% of her male offspring carrying 1 copy of the B gene. (They would be dark barred on whatever colour you are using because they are only carrying one copy of the gene.) If you then crossed one of those males back to his barred mom, you would end up with more barred birds, male and female, to work with, also some light barred males that would, conceivably, be now carrying two copies of the gene.