John, I'm wondering if Graham means these instead.
I've used them a couple of times but not enough to be fully conversant yet. Basically a very transparent paint, like a glaze or filter, used to
airbrush a slight "tint" to an area that can be built up in layers for a stronger or graduated effect. For a short time they can be manipulated or even removed with water.
Similar can be mixed up with regular paints using something like a glaze medium to reduce opacity, or even mixing paints or inks with a compatible clear varnish, but having a straight from the bottle product is very convenient.
Sorry for the huge picture, I lifted it straight from the shop