tl;dr: make your signup button red.
When I first joined Splitwise, I had one immediate goal: to rebuild the iPhone app. I did have one question, though, almost immediately on seeing our signup page:
Why isn’t the signup button red?
My usual complaint on seeing home pages is that I can’t figure out what to do next. Anyone designing a good homepage for a webservice should have one action for new people coming to the site and one action for returning users. Returning users are a lot more invested in your service, so you just need to make sure their action is discoverable, but new users should know what you want them to do immediately on page load.
Here, the sign up button is the first thing you see, so why would I care if it’s red? Red converts better. In our case, changing to this increased sign up rates by 20%:
There are sometimes reasons you want to avoid a sign up button entirely. ZURBlog talks about how burying their signup buttons actually increased sign ups by 350%. In their case, it sounds like most of their users needed time to familiarize themselves with the service before they were ready to sign up, so the hard sell of a big red button was driving users away.
You’ll also see cases of people telling you to pick some vibrant color that fits with your site. I don’t really agree with them. I’m going to leave it up to you to do some thematically appropriate button color if you really want – on the condition that you A/B test it against a big red button.
The Splitwise Blog 
Red typically tells me to stay away from it. Red is danger, red is to stop, red says something is wrong, think before pressing, etc. It might help the button ‘stand out’ but would prefer green
Danger is one association with the color red. Some other associations are courage, joy friendship, love, anger, power and determination. The general advice is to use red whenever you want the user to make a quick decision.