The Barclay would probably look fine in plain green, but I need the practise at lining models out so I've loaded up the bow pen with paint and had a go.
To help me, I made a little ruler from plastic that could be hooked over the tank edges - it's easier than measuring and as long as I don't lose it, it will be around for future efforts. The edge the pen will run along was chamfered to stop the paint creeping under it, something that happens if you have a plain edge.
I'm quite pleased with the results. The paint was a bit thick so I thinned it, a little too much as it turns out. Corners were painted in with a brush and then cut back on the outside edge with another brush loaded with turps. Maybe I'm a long way from a Rathbone but once weathered, it will look OK.