The color actually looks like 5ppm to me but if you go into a well-lit room and hold the test tube against the white part of the card right next to the colors, you'll get your best reading. I go into my kitchen where I have good fluorescent daylight lighting and can tell things much better. Going outside also works. Incandescent lighting which puts out a yellow hued light makes it tougher for me to read the differences.
The good news is that it doesn't matter if it's 5ppm or 10ppm or even 20ppm. All three of those levels are low enough so they do not matter. There is a big enough difference between 40ppm which is the first level that really matters. When your nitrates get to 40ppm, it's definitely another indication that it's time for your regularly scheduled PWC (partial water change)... but remember, there are many other pollutants that we cannot test for so do not rely on the nitrate test kit as to when it's time to do a PWC. Do them on a constant and regular basis... I like weekly PWC's on my tanks and the percentage of water changed is determined by the bioload in the tank. In a lightly stocked tank, a 25% PWC once a week is probably sufficient. In a more heavily stocked tank, twice a week PWC's... or if the tap water and tank water baselines are close enough, then once a week larger PWC's (33% to 50%) are OK.
For all the men out there, men have a much higher rate of color blindness than women. Here's a couple of free sites to test your color blindness. The first two are kind of quick. The third one gets really deep and takes a little while to complete.
http://colorvisiontesting.com/
http://www.toledo-bend.com/colorblind/Ishihara.asp
http://www.xrite.com/custom_page.aspx?PageID=77