ELOS Test kits - when is "yellow" considered "yellow"?

[Snappy]

Yes you need good light & when I test I always hold the testing tube up in front of a white back ground so the color reads true. As you mentioned, you will find that when it first changes to yellow it is rather a pale color and 1 more drop (.5 DKH) makes it brighter. That said whether you read it as pale yellow or bright yellow will only be .5 DKH difference so it is with tolerences but which ever way you read it the important thing is to try and be consistant and always read it that way. Once you test enough with it you can judge from the shade of the greenish yellow color how many more drops it would take to turn it true yellow and not even waste them on that test saving more for later.