While black bars on a white background are a very good combination, your barcode does not have to be black and white. The colour chosen must appear black under red light, so blue and green are possible options. The important thing is to make sure that the colour used for the bars is a pure colour, and not one that is created using a four (or more colour) printing process. You must also make sure that you never reverse-out the image, putting white bars on a black background, because this will not work.
Click here for our barcode colour guide