Got your attention didn't I?
We call schnauzers with two distinct colors, one on the outer hair and one after clipping "two toned dogs". Brindle dogs often appear to be two toned as well when clippered, especially if the clipperwork is done fairly short, because the brindle hair is usually lighter than the furnishings and will appear to be solid, whereas the body hair is lighter and the stripes appear to be more vibrant when clipped.
You need to remember that terrier hair is really "deisgned" to be handstripped not clippered. The hair is darker on the longer, harsh coat and the softer "undercoat" is usually lighter. Short clipping will remove the harsh outer coat, leaving nothing but the softer light hair behind.
Usually it's not a huge problem, and not that noticable, when we groom the terrier t ype coats. Most ev eryone has consistent color all the way to the skin, but every once in a while a dog comes along like the one at the top of the page, that looks like two different dogs are occupying the same body. Harsh, dark out coat, then when you start clipping your eyes start to bug out and you take a deep breath. Betcha you eben check to make sure its not a 40 on the clipper (I know I did!).
This type of coat can lead to dogs that look as if they have not been blended well even though they have. It is also almost impossible to get a smooth, straight line. You see every wobble. Every divet. The more you try to fix it the worse it gets. It is one of the things that makes me pull my hair out as a groomer. Notice that in this closeup shot the blending is definately done. I did not draw a line.
By using a longer snap on comb, like a 2 or longer or a skip toothed blade (usually a 4) you can minimize the distinct color change. If the owner wants a sh ort back however, there is nothing you can do to help. Many folks get used to seeing their two colored dogs and think if they do not look like they are used to seeing them that you didn't go short enough.
Unfortunately, there is nothing you can do to fix it if they want it short. Nothing you can do to minimize it. Just make sure the blending is done, it looks decent to the owner and let it go.
If you can convince the owners to handstrip these dogs then you, as a groomer, will be happier with the results, but if you can't, you just have to learn to accept it for what it is. A dog with a funky coat and an owner who is happy with the results. That is really all that matters in the long run.