I was standing outside the cubicle in a gents toilet recently, waiting for the occupant to exit. I knew there was someone in there, as the little sign was red. I waited, and waited. Eventually my English reserve was overcome by human need and I politely knocked on the door. Which creaked open to reveal that the cubicle was empty all along.
You see, the designer of this particular lock and integrated display had decided in their wisdom that the word 'Vacant' was to be displayed in red, while 'Engaged' should be shown in green. All that had registered with me was the colour, which I saw as soon as I walked into the bathroom. It would never occur to me to check the wording; after all, red means stop, green means go. Everyone knows that.
Except this designer, who had completely ignored the convention of what those colours mean. The fact that colours do implicitly impart meaning shouldn't be lost on designers, whatever field they work in. Some colours' meanings are perhaps less clearly defined, but green and red - especially when used together - are pretty clear.
As user experience designers part of our job is to remove doubt from the user's mind. We want to smooth the progress along their chosen path. If I want to make it as easy as possible for people to buy things from your website, for example, then I'm unlikely to make the 'buy' button red. Red means 'stop' and even 'danger', which is the last thing I want your potential customers to be thinking about at this crucial point in their journey.
Thankfully, most examples of poor usability aren't quite as stupid as the 'red means go' one I've described here. But especially in the world of e-commerce the difference between mere survival and actual success can be as subtle as the naming, placement and yes, even colour of your 'buy' button.