I have one of those in my tank as well. It seems to match what my ammonia kits read when I do a test. I put mine in while my tank was cycling and sure enough it changed 3-4 different colors over the course of 3-4 weeks. I will say that the colors on the circle don't match exactly, so you'll have to use your own judgement if it starts changing color.
Mine has stayed a light yellow color since the cycle was complete and I hope it stays that way.
"Okay brain - you don't like me and I don't like you, so lets just get through this and then I can continue killing you with alcohol" --Homer Jay Simpson