I respectfully disagree -- the two most common colours in smooth and longhair doxies are red and black and tan -- these are also known as the "classic" colours. These are also the base colours that modify the colour patterns (brindle, merle, etc) in the breed. Red and black and tan are also dominant colours. The only colour really considered rare in the longhair is sable. Wirehair is a bit different, it's most commom colour is wild boar.

We only have two longhair doxies that come into the shop, and both of them are black and tan..

Personally I think people should avoid writing "rare" in an ad. It makes people more interested in the rarity of the dog rather than the dog itself. Visa has rare lines, but I sure as heck wouldn't put that in an ad.