Silver brown dun or silver bay dun?

[Malakai10]

Is this horse silver brown dun or silver bay dun? I originally thought her a silver bay dun, but then her foal turned out seal brown.



Here she is as a newborn, if it helps:



This is her grey on seal brown foal:

[nyraeia]

Is this horse silver brown dun or silver bay dun? I originally thought her a silver bay dun, but then her foal turned out seal brown.



Here she is as a newborn, if it helps:



This is her grey on seal brown foal:

You were correct, your horse is a silver bay dun.
Now here's why the foal is brown:
A horse has two copies of each gene, one from each parent. It randomly passes on one of these copies to each foal.
There are three "versions" of the Agouti gene:
a = not Agouti (this means your horse is black)
At = brown
A = bay
A+ = wild bay

a is the "weakest" and A+ is the "strongest" of these. We call that relationship "dominance". At (brown) is dominant to a (black). A (bay) is dominant to both At (brown) and a (black).
This does not mean that a bay horse cannot also have a "brown" copy. It just means that both brown and black would be masked by the "bay" copy.
Your mare is bay. This means, she has at least one "A" gene. Now, the other copy of her gene could be either another A (which we would call homozygous), At or a. It doesn't show because both At and a are recessive to A.
Could the other copy be A+? No, because then she'd be wild bay - A+ is dominant to A so it would mask it.

The foal's sire is cremello, which is red-based. Just from looking at him, we can't say which Agouti copies he has because Agouti is not visible on red-based coats. Without looking at his progeny, he's basically a surprise bag.
We can only be sure that he isn't A+/A+ (he would have given "wild bay" to the foal which is dominant to brown) or At/At (he would have given "bay" to the foal) or A+/At (both of these would have been dominant to brown).

So why is the foal brown? We can conclude that it has to be either At/At or At/a, because everything else could not result in a brown horse.
This means your mare cannot be At/At because then she would have passed an At to the foal and the foal would be bay.

Your filly got "brown" either from her dam (where we only know one copy of the Agouti gene for sure) or from her sire (where we know none of the copies for sure).

I hope you can understand this, it's kinda early for me.
If you have any questions, just ask!

[Malakai10]

Thanks and it makes perfect sense.

[nyraeia]

Thanks and it makes perfect sense.

Great! :)