bagof4grapes, if your horse had the dun gene, wouldn't he be a grulla? Are you arguing that he is a very minimally expressed grulla?
I have an essay to write and feel the need to procrastinate so I thought that perhaps I could help explain a few of these colors. I found all of these pictures through Google and am using them without permission, of course. Links to where they came from below the pictures.
(I have my fingers crossed that Chiilla wont tell me I've messed the colors up too badly xD )
This is a black horse. Note that the muzzle and flanks are consistently as dark as the rest of the body.
(The Horse Guide: Black Horse Color
This is a black horse that has faded in the sunlight.
Here's a brown/seal brown horse. Note how the muzzle and flank are conspicuously lighter than the rest of the body. Horses of this color can be a dark black with the exception of the mealy points.
Here's a lighter or perhaps sun bleached seal brown horse. I couldn't find a very good example of this one but I know some of the forum members have brown horses that bleach like crazy in the summer.
(Seal brown (horse) - Wikipedia, the free encyclopedia
Here's a black horse with a dun gene (aka a grulla horse.) This is the color that you are saying your horse is?
For reference, a horse that I'm pretty sure is a darker grulla. See how the body color is still more of a silver shade than brown?
(this image found on the horse forum through Google. I can't find out where it came from originally)
Here's a horse that, according to the website, is a brown based dun (aka a seal brown horse with the dun gene.) They apparently look a lot like a bay based dun.
(Dun Central Station - Brown/Bay Dun Colors & Markings
I suppose that you could argue that your horse is black with one copy of the creme gene (smokey black.) These horses are a dark choclate color but do not have any mealy points. I should note that a seal brown horse with one copy of the cream gene looks like a dark buckskin.
(Smoky black - Wikipedia, the free encyclopedia
Keeping these pictures in mind, do you see why everyone has come to the conclusion that your horse is a seal brown? I'm not trying to force my opinion or anything. I just thought that being able to see for yourself what the various colors in question look like would help you to understand what everyone is talking about.
I don't know very much about what horses look like when they're clipped except that you can expect them to turn bizarre and entertaining colors. What a horse looks like when it's clipped can vary tremendously even amongst horses of the same color. The first rule of equine color genetics is that you can never tell the true color of a horse when it's clipped.
It's certainly not a bad thing that your horse is brown. That's one of my favorite colors. It's even cooler that he turns such a neat red color when he's clipped. Think about it: you could have a black horse in the spring/fall, a brown horse in the summer, and a red horse in the winter!