oops. this all means that my homemade 4dkh solution is wrong, it is green as soon as I add 2 drops of BB. Tap water turns blue immediately, so I don't think its the fault of the bb solution.
Its so hard to test my 4dkh, my api KH test kit is so hard to tell, it just says drop by drop 'til it turns bright yellow', but 'bright yellow' is rather subjective, the difference between 2kh and 4kh is impossible to tell. Back to the drawing board. Might just pay for some 4dkh solution. I hate paying for what is fundamentally water at the end of the day! But now is not a good time, as I've just set up my co2, just planted a whole lot of plants, and in the evenings, my fish have been gasping. grr!