Another note about sorrel: It's a colour that's only recognized by the AQHA and the APHA, so that's why it's usually only a Western term. Any other breed it would be called chestnut.
Actually, it's recognized by the TWHBEA as well. We had a mare that was registered as a sorrel and one that was registered as chestnut and the difference between them was the shades of red and one was more of a coppery hue than the other one.
I've heard chestnut horses with flaxen manes and tails called sorrel, but they are genetically the same as far as anyone has found. So I just call them all chestnut, but a lot of people like to be REALLY specific about their horse's color, whether for breeding reasons or just because. For example, this would be sorrel:
LOL, it only really gets confusing when you do start trying to differentiate between the shades. What I call chestnut, others might call sorrel and vice versa. Basically, any horse with the genetics 'ee' is going to be some shade of red. Chestnut is a term that is a catch-all for all red horses and anytime you use the term "chestnut", the other person will know that you are talking about a red horse with ee genes. To get more descriptive is where the phrases "flaxen chestnut" or "liver chestnut" come into play .
I just have to say I love liver chestnuts. I used to know this little pony when he was a foal bright red but I saw a picture of him awhile ago (he'd be about 4/5 now) and he was a gorgeous deep liver chestnut.
Me too, we used to have a stud that was a deep liver chestnut, even darker than the one in the link I posted. The only way you could tell he wasn't black is by the faint copper sheen he had in bright sunlight. I wish I could have gotten at least one foal out of him, he was stunning.
Flaxen sorrels/chestnuts are have the flaxen modifier that only affects sorrels/chestnuts. Calling a flaxen chestnut a sorrel would be wrong if the "flaxen" was left out. It'd be like calling a silver bay just a bay.
What causes the differences in shade is unknown though. Same with any other horse colour, as in some buckskins are deep gold while others are buttermilk and bays come in a range of shades too.