I am considering buying a cairn terrier. The last one the couple has is wheaton colored with black tips and muzzle. There is little white or lighter coloring around the nose and on the chest. I am buying a pet not a show dog. Does the coloring really matter?

Doesn't matter. Many of the brindled dogs change color over their lifetime so its best not to get too attached to one color over another anyway. The breed standard allows any color but white and mentions the black points as 'desirable.'

Emma is registed as a "wheat brindle", and was very black all over untill she got older and she lost it all, however she still has black points on her face. but color wont matter if you dont show.

First cairn was a black brindle who went to wheaten and finally settled on all gray. Current was a black brindle puppy who is now 1 year old and black/wheaten brindle with a red muzzle.