Watercolour Marker Pens! They have a brush at one end, a felt tip at the other, and a pot of ready-mix in the middle! A bargain in Cass-Art (especially when they generously price-match the same product online). I also got a water brush pen to use with them. Use it to prudently wet the paper before, or after to worry the paint about. I did a quick (30m) Jaguar D Type to try these out. They are a LOT more portable than brushes and could lead to some more ‘al fresco’ painting. I think they call it ‘plein air’?