Yellow is the best color for this. It's bright and catches the eye, plus the yellow ribbon project is an awareness campaign about using the color yellow to indicate your dog needs space and shouldn't be approached (for whatever reason). In my experience hardly anyone knows about the project, but that just means I have the opportunity to educate if I happen to be using it.

Some items that could help without messing with the public's perception of service dogs:
"Nervous" vest
Velcro Do Not Pet patches
Velcro Nervous patches
Do Not Pet bandana
Give Me Space bandana (this one isn't yellow but is still pretty good)