Rare and Common

Just goes to prove the part of the country your from makes a difference. Grulla is pretty common around here.
It would be hard to say what the most common color around here is. Other then the hand full of other barns I have been to, I don't really interact much with the horses in the local area. People here do seem to like colored horses. For example on my way home from town I pass (in order)
-a BW paint and a blue roan paint
-2 roans and a grulla
-a bay and a chestnut
-a grulla overo (weird right I had to stop and talk to the old man that has him)
-a chestnut paint
-2 buckskins and a red roan appy

In my part of the country, we are primarily QH and paint territory (And being a western rider, that's about the only breeds I ever see). I said grulla because that's what I've always been told, but seeing that there are quite a few breeders that breed for the color, I doubt that is the case anymore.

P.S. I've seen a grulla paint, too. She was so odd looking. Her mane started at the top layer as grulla colored but turned white on the underside. Pretty cool, though.

I'd say bay was most common in my area and I mean realllly common.

As a note, albino has never been found to exist in horses, so if you're seeing albinos then yeah, they WOULD be the rarest!

I was going to say I've never seen/heard of a cremello or perlino dun, but I did a Google search and it seems the double cream gene just hides the dun anyway, so nothing special there!

Brindle would be the rarest pattern, but it's not a color. Most brindle horses are a mix of two very common colors actually. True tobiano (with no other patterns) would probably be a close second!

I really don't know if dominant white would be the rarest, unless you're referring to the completely white horses. Dominant white can display as totally white or partially white, so as a genetic gene it's a bit of a toss up!

Chestnut is definitely the most common around here, followed by bay.

Of the more 'common' colors, black and gray are pretty rare. Obviously the more complicated colors are always going to be the rarest, such as palomino, buckskin, grullo, roan, and champagne. And the even more complicated ones, such as cremello dun, palomino roan, and champagne cream will be even rarer.

Around here I'd say probably Bay then chestnut would be the most common color... and I've never heard of a Dominant White (or True White) in our area, so I'd have to go with that as the rarest (since it's also rare elsewhere in the world too)

That is exactly what I was thinking! There's dominant white, and maximum sabino. Both look albino, but genetically aren't. I did some research on genetics recently, and some scientists recently (within the last month or two) put out some info on albino. If I remember correctly, they said that they discovered a way that horses could be albino(it was thought before to be 100% impossible), but there has never been any record of it. They also said that the creme dilution gene results in a partial albino. So Buckskins, Palominos, Perlinos, Smokey Black etc are technically partial albinos.

Anyyyyway, I live in Thoroughbred country, so it's mostly bays and chestnuts. However I'd say bays are a tad more common. (By the way, for whoever it was that had asked if bays were dominant, yes, they are. It's the agouti gene over a black base coat. Agouti is dominant to black, and technically it's dominant to red too I believe, but since it doesn't act on a red base that doesn't really matter...) Greys are also extremely common around here. Paints/pintos and Creme colors (Palomino etc.) are also fairly common. Rarest color (excluding patterns, like Brindle) would probably be grullo. I've only seen one I think in the last 9 or so years.

Actually albino in horses is really common. You see it all the time - cremello, perlino and smokey cream. Albino is not required to present as snow white with pink eyes. Cream hair with blue eyes is still albino.

Sort of... There's two different variations of albinism. Cream horses are one form, but not technically the "true" for of albinism that most people think of. There's more info here if you're interested in it. Albino Miniature Horse

Here in the area that I am in, I would have to say Bays are the most common. But I live in/around Mennonite country, and a lot of their (buggy) horses are Standardbreds. But, the barn I ride at is full of grey ponies. Seriously, I think the BO has an obsession!

To be honest, I think the area I live in has a pretty good mix of everything. Although there are a lot of English riders, there are also a fair amount of Western riders as well. All the barns seem to have just about one of everything.