When I was trying to pull this project together in my head, I decided that I needed to produce each canvas in response to it's surrounding. A character lost and cold in a big city would act and look differently than one lost in the woods for instance. So what I needed were canvases small enough to fit in my pockets and paints. The canvases were easy enough to find, but I became stuck on the paints.
Until I wandered into Soho Art Materials (just off Broadway on Grand in NY) and the lovely Greta recommended these DecoColor pens. They are probably the first "paint" pens I have used that produce reliable results.
The colours are really bright and they HAVE got fine nibs that last, not like some pen makers whose "fine" tips are like drawing with the blunt end of a yard brush. One word of warning though, don't get smashed on red wine and push down on the tip too hard or too long, as this will cause the paint to piss out all over your sodding work!
Sadly as yet I haven't found these pens anywhere on my travels in the U.K...