Uniform question

This is a minor thing and just a point of curiosity for me since I'm American and don't know the British uniforms.

It seems that Thomas wears a slightly different uniform from most of his fellow officers. I haven't had a chance to really compare the jackets closely, but they seem to be the same outside of the usual rank insignia and medal differences. Thomas, however, wears a dark green/brown shirt with a tan tie, while the rest seem to wear a tan/brown shirt and dark tie. At least by American uniforms standards, it stands out as odd, so I assume there's some specific reason for it.