I varnished my Soviet Shermans this morning and saw most of my hard work disappear. This is the result. There is still some sign of the pigment, but it has darkened rather spectacularly and has almost become invisible.
I was sent a link to this tutorial done by Piers on the Guild. He recommends using a pigment fixer rather than just mixing them with water. So the trick appears to be, varnish, add fixer, then add pigment. I shall know for next time.
Sadly, none of the local hobby shops appear to have fixer in stock, so I think it may be a while before I get a chance to rectify my mistakes. However, it's been a learning experience and I'm glad that I at least gave it a try. However, this being sick nonsense is really beginning to get on my nerves.