Amazon did extensive research into this and after a LOT of testing, made the add-to-cart buttons yellow. This proved to be the colour that psychologically "encouraged" shoppers to add to cart. Yellow with blue text also works well - PayPal uses it - again, their testing showed it to encourage the highest responses. Green is too "passive" and red is a definite NO - as it suggests "caution" or "danger".
Website colours are also vitally important, with good tone compatibility being essential. Blues work well - and dark (royal and navy) suggest a corporate sophistication. Browns and greens are too "earthy" and some people see these as "dirty" colours. Bright colours can clash, and be distracting - especially where there are calls-to-action (such as Add To Cart, or Checkout).
On our main site, for a long time, we had cream-ish coloured buttons with a soft grey text for call-to-action events. We changed our corporate colours to deep blues and yellows, and our call to action buttons are now deep blues, with yellow text. We noticed an increase in responses to call to action functions, including add-to-cart and checkout.
The more "corporate" a site looks, the more comfortable people feel when using it. Don't go for colours YOU like... use button colours that have proven to be good at getting people to click them.
