Shane,
I really think it varies from tank to tank based on that tank's specific parameters: lighting, c02, current, bio-mass, fish load, type fish, etc.
IME, I just had my DC at an amber color for a day or two while I was adjusting my new reg, and I saw NO adverse effects on the fish.
No gasping, stress, nothing. But I do have a lot of current/flow.
However, I have read multiple threads where folks reported their fish were having issues at yellow or less.
To me, I would not want it any color that blue or green to err on safety's sake.
Hope this helps.