Wardly Nitrite Test Kit
Well I'm not sure I would call this a professional test kit -- but it is a very easy one to use if all your interested in is a "Yes" or "No" result to "Is Nitrite present in my system?". All possible colors are some shade of yellow which can be hard to tell the exact measurement. If no Nitrite is present in the water then you will get a nice clear test sample.
Overall this is a real simple kit to use: Take some sample water, add 5 drops of reagent 1, shake, wait a minute, add 5 drops of reagent 2, shake, and right away you check the color value.
I'm not a big fan of "Wardly", in general much of what they make is cheap or not good enough for reef level testing. But this kit seems to work well enough. When it comes time to by another kit, I may get a different brand.