If the GH and KH are above zero, the first drop will turn GH very pale orange and KH will turn very pale blue. The colour will get stronger with each drop till it changes (GH orange to green; KH blue to yellow).
From the pic above, it looks like your GH is green and your KH is yellow - that's after 2 drops, and they are same colour as the first drop? The colours are always very pale until you've added quite a few drops.
Just to check they haven't changed things since I bought my API testers - does the liquid in the bottles look a sort of olive green for GH and orangey yellow for KH? And the first drops turn the water a washed out version of the colour of the bottles?
Did you give the bottles a good shake? I know it doesn't say to, but I always do.
If that's what happened, you have very very soft water (GH is zero) and no buffering capacity (KH is zero). Does your shower ever get limescale on it? If you really do have zero GH, there should be no limescale.
The thing I'm a bit confused about is that if your KH is zero, it should be very easy to drop your pH with chemicals; there is no buffer to bring it back up like you said in your first post. In fact it should be so easy to drop your pH that you are in danger of a pH crash - that means the pH dropping a long way very quickly, which isn't good for fish.
Could I ask you to try something? Add a tiny bit of baking soda to a tube of water, shake till it's dissolved then do a KH test. Baking soda contains carbonate, and that's what the KH test measures. That should make the colour go blue with the first drop so it will check the tester is working right.
Assuming that the tests are right, and both GH and KH are zero, you are in an unusual situation - very soft alkaline water. You would be safer keeping fish that like soft water, even if they prefer acid water, rather than fish that like hard water. It is easier for fish to cope with the 'wrong' pH than it is for them to cope with the 'wrong' hardness.
I think
Does anyone have any different suggestions?