Colour doesn't really exist, yet football teams with a red kit are statistically more likely to win than teams in other colours (tell that to Chelsea fans); coloured placebo pills affect how effective the pill is (apparently red makes us more sexually receptive and blue more relaxed, um Viagra?); percieved colour changes completely by what's next to it (turn grey to yellow by changing adjacent colours); and Isaac Newton randomly named the colours of the rainbow.
Cultural responses to colour differ substantially, but they are still common associations and and influence mood and expectation. It's all in the mind but then that's pretty much where everything lies.
A great article exploring facts and fiction of colour.