Join Date: Feb 2019
Location: Los Alamos, New Mexico
Every pH indicator has its own range. Bromothymol blue is popular because its range is pH 6 to 7.6, which is just about right for most aquaria, and it is fairly stable, relatively nontoxic, and not too expensive. High pH tests use cresol red with the range 7.2 to 8.8. Outside these ranges, the color saturates.
If you want to use cabbage indicator, you'll have to calibrate it to get the color transition at the right CO2 level, which may mean experimenting with the dKH of the dropper solution (4 dKH may not be right for cabbage indicator.) And the indicator is an anthocyanin, a natural compound that is not terribly stable, so it won't last long in the bulb. But if you plunge ahead and figure out the calibration, I'd love to hear about it.