Make sure with each drop added you give the vial a good shake to see if the colour changes, it shouldn't be keep still whilst adding drops one after the other, you need to spread the concentration of the chemicals throughout the water as the rating is based on testing a full 5ml or whatever for your tests.
Saying that I am sure you do this anyway, as it will be clearly stated in the instructions
Also just to re-iterate you are looking for the transition from one colour to another, not the colour matching a card...orange to green shouldn't be too tricky to see happen, they are contrasting colours
I'm off to get my 4 x Tapajos shortly as well as some new plants probably...I have just moved some old media (used since I first started the hobby in October) and 4 peppered cory from the main tank into the 125L one, fingers crossed there is no mini-cycle...I'll be testing it everyday until I am comfortable that the bacteria is doing it's bit. If I have to I will be doing twice-daily water changes to get over any bacteria issues and I'll just move the tapajos into the main tank and hope there is no ich issues etc. Should be okay but as this is the first time I've done this I am a little weary
Cheers