What I do is I put the bottle infront of the white portion of the card, and then use my other hand to close one eye. I find it more accurate to judge the color. I'm not too sure what to do about the colorblind part though...
cheers guys, when I measured an established tank it was so similar to this reading too. Took a leaf out of your book livewire and just asked the girlfriend, she said it's pretty much zero reading, and almost identical to the tank that's been running for a year or so. Just got home from work to about 30 babies in there also so the conditions can't be too shabby