Yeah, if we could see what you mean by light and dark brown on the tail, that would be great!
Gerbils, like some other animals, cats, rabbits, guinea pigs, for example, can be "color pointed". A type of color point cat is the "Siamese" (where the name for the gerbils come from. Burmese cats also exist, but their color points are not very distinct, like in a Siamese). Basically, what is happening when a cat (or a gerbil) has two copies of the color point gene, all pigment will be washed out, except in the coolest parts of the body, like the tails, feet, face and ears. You will notice that these color pointed animals will get darker in the winter, for this reason. Now, in gerbils there is Color Point, and Light Color Point. In cats, there are Color Point, and Dilute Color Point.
Let me illustrate this for you.
If you took a BLACK cat, and gave it the two color point genes, you would end up with what is called a "Seal Point"
here is a picture of my Black cat, Stanley, with my Seal Point Siamese, Frank
Just like with gerbils, the points color is faded a bit with the CP gene. His mask, ears, tail and feet are now dark brown, and not really black.
Now, if you were to take a black gerbil, and make it color point, you would get a "Burmese"
Now with cats again, if you were to take a black cat, dilute it (which would turn it "blue", or grey), and then give it the color point genes, you would get a "Blue Point"
this is a picture of my blue point Siamese, Joyce (and yes, I'm wearing feetie PJS)
Now if you were to take a black gerbil, and add one color point gene, and one light color point gene, you would get a light color gene, you would get a Siamese.
For a few more examples...
What if you took a brown cat, and made it color point? You'd get a Chocolate Point Siamese, like my cat Raymond, who is actually sitting on my lap right now!
Other types of CP gerbils?
Well, you could basically take ANY gerbil color and "make it" color point. Anytime you make a Red eyed gerbil color point though, you will get a PEW.
Color Point Nutmegs...
Also, many times you make a gerbil who is A-ee, you will get a black eyed white.
For example, my old CP Polar Fox, Marilyn, who also turned out to be spotted!
You should also know, that when you combine two c[h], you will get a Dark Tailed White.
I hope this helped!