As far as I know all pH test kits that measure pH from about 6 to about 7.2 use bromothymol blue, so you can just use your pH test kit reagent.

Mix 6 grams of bicarbonate of soda (baking soda) into 1 liter of distilled or DI water, giving you 200 dKH water. Then take 10 ml of that and mix it with 490 ml of distilled or DI water to get 4 dKH water. Add enough of this water to the Red Seas CO2 Indicator to fill it to where they say it should be filled, and add 2 or 3 drops of pH reagent to that, to get a good, easy to see blue color. The amount of the reagent you use isn't critical at all.