Cro,
It is absolutely impossible for RO water to have a KH of 20. Either you don't have RO water or your test kit is complete rubbish. A KH of 20 means that dissolved in that water is the equivalent of about 360 milligrams per liter of calcium carbonate. One of the main benefits of using an RO filter is that it totally removes carbonate ions from the water. So not only is this impossible it's an absurdity. Your test kit needs to be immediately tossed into the rubbish bin ASAP.
The dropchecker article states that the fluid must be RO water, but the water that comes out of the RO filter at exactly ZERO KH - and is then adjusted to a known kH value by adding carbonate back into that water. That means that a sufficient quantity of carbonate or bicarbonate compound, such as baking soda (sodium bicarbonate) is added to the water until the concentration of carbonate is 71 milligrams per liter, which equals a dKH of 4. One can add enough for a dKH of 5, or about 80 milligrams per liter. "Known" means that one knows exactly whether the value is; 4 or 5 or 6 or whatever. The color changes will vary based on that KH value that's why you have to "know" what the value is.
As Aaron and others have pointed out, if you will not use the 4 dkh water then you may as well not bother using the dropchecker at all because it will stay yellow until the cows come home. This is like having a non functional petrol gauge on your car and then deciding to use it to determine how much petrol is in the tank.
Cheers,