1. Red is the color of sun, warmth, life. It calls for action and it has a provable effect on metabolism: it increases blood pressure and speeds up the pulse. Red has the longest wavelength in spectra which can be seen with human eye. If the wavelength is even longer, it can be sensed as a heat. In this case, it is called infrared.
2. The color red is the color of passion, seduction, and lust. Ever heard about red light districts?
Redheads were considered very special through all known human history. Women with red heads are considered more passionate, have more intimate partners and are in general more popular among men. Many ladies who colored their hair red reported after that simple act their life really changed and became more similar to this stereotype!
There was even a research in Germany made in 2006 which proved redheads have more sex than brunettes or blondes, but this stands only for ladies. Men with red hair were never popular...
One possible explanation is the rarity of the gene for red hair. Only one percent of the world population carries this gene. Men with this gene can look too risky to be considered as possible partners to women, but women with this gene can look like a trophy to men!