I've used my ink-samples to paint for a while now and I thought I'd share my 'trick'. Not really a trick, but it makes it easy to use them.
Before I open the vial, I shake it, so inkdrops stick to the cap.
When I take the cap off, I simply add a few drops of water (or more, depending on the dilution I want). I use a temporary cap to keep the vial safe (I speak from experience cough).
And then I paint. :D
When I don't need it anymore, I discard the contents of the cap, wipe it clean and screw it back. This way, the ink stays clean and I don't feel I waste much of it. Sometimes when a vial almost completely runs out -- because I have in fact put the ink in a pen shocker -- I'll add water to the vial itself and mark it for 'paint'. This doesn't always prevent me putting it in a pen anyway, but hey.