Yeah, it depends on the scenario, green is like a very peaceful-ish color, red is like something to relate with urgency (In a negative way). Red in real life means danger, stop, error..etc.

I wanted to recommend red too initially, but I think for his use case blue or green would go well because of the background image (and also for the reasons above).

This guy does a nice job explaining how sometimes it's not so much the color, but how you use it. "What stands out gets clicked, what blends in gets ignored."

http://socialtriggers.com/best-color-for-conversions/

It depends on a lot of variables. Best to A/B test which color works best with this specific design/audiance.

> Any reason not to use red ?

Orange.

If it's good enough for carrots and William the Silent, it's good enough for me!

Red is associated with "error" state.