I think only a few people/horses can pull off the grey coat. The problem is, you'll stick out like a sore thumb. Which is great if you and your horse and your equitation is perfect, but not so good if it's not - because everyone usually wears black or navy.
For my chestnut I just got a hunter green coat, which I love because it is a subtle pop of color that always looks good on chestnuts. I am also thinking about getting a chocolate coat as well.
When in doubt, get a navy or black and then use the shirt underneath as a tasteful pop of color. Maybe even a customized collar to go with it.