It is said that black dogs don't find homes as fast as other colours. It's also true that the older dog may not appeal, go as fast, as a younger dog, and puppies find homes quickest of all. So I guess if you are a black elderly dog, you won't be finding a home any time soon - maybe!!
As for temperament, obviously the one that comes forward will have more kerb appeal, to most people, than the one that hangs back and looks bored about the whole thing. There are always people who feel sorry for the one at the back of the kennel however. The more full on dog, the one that rushes the fencing, will put most people off too - although the aggressive won't be up for adoption in any case.
Small to medium dogs are going to appeal to more people, especially these days when people are strapped for cash (feed).
So the black, elderly, retiring (or overly boisterous), big dog is also facing a long wait ??
ps Don't necessarily be put off by an elderly dog who perhaps has a medical problem. Many Shelters will help owners who take these dogs on, with the cost of any ongoing medical needs. It's very rewarding to take on a needy dog like this.