Different cables have different colors for the left channel (I have seen white, black and yellow) but red is always the right channel unless you have changed the settings to swap the channels..
You can tell quite easily once you plug it in...just go into AudioHQ, double-click on Speaker, then click on the Test button under the speaker setup. The audio will tell you which channel is being sent information. If the sound corresponds with the placement of your speakers, then you've got it set up correctly, if not, then you need to swap the plugs.