Why the Color Red is So Weird in Design
Summary
The color red has an odd affect on the human brain. It might help us in some cases and scare us away in others. Our relationship with the color is complicated enough, that saying red is a better conversion color is simply false.
The Science
In a 2005 study of the Olympic games, researchers examined one-on-one sports, such as wrestling, boxing, and Tae-Kwon-Do. They found that a majority or winners were wearing red uniforms, as opposed to blue, winning 16 out of 21 matches.
In other studies, red was associated with higher testosterone levels, anger, strength, and more. The effect is subtle, and only had a significant effect when participants were relatively evenly matched, but placed those with red as their defining color at a slight advantage in athletic, strength based, competitions.
In another experiment, researchers examined the effect the color red has on academic (IQ or information) test taking. They split test takers into 2 groups. One group was exposed to the color red before or during the exam. The other was not.
In 4 different sets of studies, researchers found that being exposed to the color red consistently lowerd participant's test scores and led them to do worse without their knowledge. The color red, associated with danger or excitement, distracts the brain from intellectual tasks, instead making it prepare for a different sort of pursuit entirely.
As to the question of which color converts the best, study after study try and show how red, or orange, convert better than blue or green, for example. These studies have since been shown to mostly be false, and usually include a missed base color tipping the balance. The biggest deciding factor for an action button is not its color, but how that color contrasts with its environment.
The Practice
It's clear that the color red might have a positive effect on a product's customers in some cases, and a negative effect in others. From these studies, which I have heavily abridged here, we can construct two main rules of design.
1. The color red is not a better conversion color than any other on the whole. The majority of its impact is in how red might contrast against its background. This effect can be replicated with any other color just as well.
2. The color red does have an effect on certain types of activity. It gets your visitors in a certain mood, and that can be great in some cases, or a disaster in others. Use red in a sports website to get people fired up and loaded with testosterone. That's great. But if you use too much red in your kids site, you're just looking for trouble.