See customer image for instructions sent with my order on 9/9/11. Both colors have their own pH scale and could be used independently. It's like a double-check when using both colors. Using just one color can be tricky with subtle differences in shades, but both colors help you nail it. It's really cool, way better than standard litmus paper!
The container could be better designed and is difficult to open at first, but if you intend to use these regularly just lightly close it without pushing the cover all the way down and you will be fine. I intend to purchase these again when I run out.