These paintings use lines 'etched' into wet paint. I typically cover the canvas with a single uniform colour to start with. I let that dry, then paint a fairly thick layer of a different colour on top of the first colour, sometimes using a sponge. I then have a fairly short time to etch a design into the wet paint (typically with the other end of a paint brush) letting the initial colour show through. This is followed by several additional layers. The final effect is usually far removed from the initial colour. I sometimes use wet charcoal to emphasise the etched lines.
I really enjoy painting like this and I am often very pleased with the resultant effect. The process of etching tends to be quite tense - there is an element of risk, especially if I have invested considerable effort into the background. Etching has to be done very quickly (especially for a large canvas) and it is hard to correct mistakes.