Hi battler,
This is were the difference between a base pigment colour and other genetic modifiers come into play...
Think of each pigeon having a ladder (the dna strand) that has many rungs on it(alleles). Each rung determines a certain character or feature of the pigeon.
There is a rung (allele) on the ladder (dna strand) that controls what base pigment colour a pigeon will be. On this rung a pigeon can have one of three options. Brown, Wildtype(Blue/Black), and Ash-Red.
No matter what any of our pigeons look like they all have this underlying base pigment colour and it CAN ONLY be one of the three colours mentioned above.
So how do we get so many other colours and patterns in our pigeons, like the white bars?
There are many other rungs on the ladder that contain other genetic modifiers that affect the end resulting appearance of the pigeon (it's phenotype). And a pigeon may have many of no other genetic modifiers within its dna strand (ladder).
Depending on what other genetic modifiers a pigeon may have this will effect the appearance of the bird according to that particular modifier. For example you can have Brown, Wildtype (Blue-Black), and an Ash-Red pigeon all carrying the gene for Hom Recessive Red and all three pigeons will appear to be a reddy brown colour all over, even though their base pigments are different, as Recessive Red covers up the base pigment colour (like a pigeon wearing a jumper).
So to have white bars, a pigeon would need to carry a gene that creates this effect on the pigeon...and this gene is called Dominant Opal (Od). A bird can be any base pigment colour and still show white bars because it also has the Dom Opal Gene.
A very good example of this type of bird can be found in the current thread jimmyrenex has opened here in General Discussion:
http://www.pigeons.biz/forums/showthread.php?t=13322
The bird in the fourth post carries Dominant Opal and has white bars.
Regards
Alaska