There is a new design drop checker for sale on the internet: CAL AQUA LABS - Double Check CO2 Checker. It is unique in having two compartments, one with the usual 4 dKH water in it, and one with what is probably pH6.6 buffered water, both with bromothymol blue in them for the color. This gives you a reference color to match, right adjacent to the regular drop checker, exposed to the same lighting conditions and in the same shape container. I just ordered one to see how well it works. I plan to report on it here when I get it.

The price seems high at first, but it includes two different solutions, and shipping is included, for now at least.

That looks really nice, doesn't seem that pricey really.
Because the reference solution is still exposed to tank water/gases can we assume it's non reactive and therefore not reagent?
I wonder what it would take to replicated it with 2 diy drop checkers.
Could you simply make a sealed container and adjust it to the right color?
Would that degrade and change over time? (you could just re do it as needed I guess)

The goal shouldn't be to put a given color liquid next to the drop checker, for comparison, but to put a known pH sample with the same amount of the same reagent in the same size and shape container next to the drop checker. Then anything that affects our determination of the color of the drop checker also affects our determination of the color of the reference solution. Since I have no more information about the CAL AQUA device than what is on the web page I don't know for sure what is in the reference bulb. It is just logic that tells me it must be a pH6.6 buffered solution with bomothymol blue reagent. (And, that is what I planned to use in my DIY design, two chamber drop checker.)

I'll admit that logic has to be bashed into my head sometimes,
That said, a known ph sample, exposed to co2 gas will change color, unless the reference is unaffected by co2 levels.
If you look at the pictures they provide it's clear that the reference color remains constant. That makes me conclude that the solution they provide is unaffected by co2.
So if you make a reference solution that is 6.6 ph (kh is irrelevant) and seal it from any influence then it will serve the same goal, no?

"Then anything that affects our determination of the color of the drop checker also affects our determination of the color of the reference solution."
...well anything visual, like lighting, shadows, whatever.
I just tried adding lots of kh to a sample with reagent, as I guessed this did not change the ph and therefore the color, so would vinegar work? Edit; This is an example of my thick head, increasing kh also increases ph right? So I was going in the wrong direction with that.
What would be a stable way to acheive 6.6 ph solution?
Maybe we are saying pretty much the same thing. So if we agree that 6.6 ph solution is needed, what is the best way to get it?

A buffered solution buffered to 6.6 pH is what is needed. Those solutions can absorb a limited amount of acid or base without changing in pH. But making such a solution isn't a simple matter for non-chemists such as me. You can easily buy buffers that hold a solution at a constant pH, but I haven't seen one that holds a 6.6 pH, 6.5 is the closest I have found.