And to confuse matters even more there are actually two types of bromothymol blue. Protonated and De-protonated.
De-protonated appears as a blue liquid before adding to ?dKH water.
Protonated appears as a red liquid before adding to ?dKH water. (Looks red in the bottle but is actually concentrated yellow and stains yellow on a blot test).
I put ?KH above because they act the same way no matter the KH of the stock solution water. I have done side by side tests using the same KH of water, the standard 4dKH and my tank dKH which at the time was dKH9. The water I used at 9dKH was my water change water before adding GH salts, it is reconstituted RO/DI using KHCO3 at 8.1g per 25L, the reason for doing this is that according to the ph/KH/CO₂ chart the curve is quite flat here and so the colour change is more gradual and more accurate than a lower KH which indicates in a narrower window.
I found that regardless of the dKH of the stock water solution Protonated Bromothymol blue will indicate 30ppm CO₂ well before the De-Protonated does if the advice you are following is a green drop checker meaning 30ppm CO₂. If this is the advice you are following then with Protonated indicator you will be underdosing CO₂. If you go by a yellow dropchecker for 30ppm CO₂ using De-Protonated indicator for 30ppm then you're overdosing CO₂.
If using Protonated Bromothymol Blue you want a yellow dropchecker for 30ppm.
If using De-Protonated Bromothymol Blue you want a green dropchecker for 30ppm.
Both the dropcheckers on the left were photographed immediately after removing them from the tank, the checker on the right is a control which never went in the tank and is the colour for atmosphere equilibrium (both liquids indicate this colour for atmosphere equilibrium when added to whatever dKH water). Dosage for the indicators was at 3 drops per 5ml of dKH water, this test pic was 4dKH stock solution. The differing size of the dropchekers above made no difference when compared to a later test using both same sized dropchekers (not photographed), the reaction was the same.
Using a lower concentration of indicator say one drop per 5ml narrows the inaccuracy window but makes it very hard to read.