Our rescue dog has settled well, but he became very upset when I put on a green coat. Why could this be?

Almost certainly, the coat triggered some unpleasant experience from the past. Perhaps his previous owner had a similar coat and wore this coming home, before mistreating your dog. It can be a problem with some rescue individuals, because usually you will not know there is anything to worry about until the situation arises, and it can have a dramatic effect on their behaviour. In this case, simply avoid wearing the coat when you want to take your dog out for a walk, so as not to upset him.

When confronted in this way, some individuals will react by growling loudly, and adopting a submissive posture, while others will simply slink away. Dogs do have the ability to memorise things in their lives.
This is why they can recognise children and other family members when they return home, even after some time away. It also means that hopefully, a dog will recall its basic training as well, when required.