gro-tek is dark brown like mud.Well the nutrients I use are a decent shade of brown, I don't know how the gro tek is but when the fluid is in the test tube it looks considerably lighter, and even with a base tint in the nutrient, it's really obvious to see how it changes reliably with the indicator drops. You just have to check it out yourself. And once you use a pH meter to calibrate what the colour should look like at particular pH's then you're golden. I will emphasise again that if you needed to go outside the range of 5.5 to 6.5 these drops would not be as useful. That is because it's easy to see a Red/Green swing on either side of the neutral Yellow, but you don't get that opposite contrast swing further down.
But for me the main reason I use drops is that I just got tired of continually checking calibration, then finding my fluid went mouldy and have to mix up another, or buy another buffer, it's was just a total pita. And the reason for this is that I'm just not prepared to go and make a radical change based on me trusting the pH meter. But that's just me. It's not for everyone. I like simple. Simple is good. The less unnecessary work I need to do the more I get to enjoy looking at the plants.
I used a pH meter side by side with the drops for a year, then when the pH meter broke, I was confident with the drops. It's hard to explain how much more satisfying it is to see what you want to know as colour rather than as numbers. It feels more connected to the plant. If you've already got a meter and most people have, I would urge anyone to get a bottle of the indicator drops for ten bucks and use it side by side for a while. You have nothing to lose.
I never got mould in my calibration solution but I did get algae.
if you store it in a dark place it stops the algea.