The tree slapper is a tidy looking plane, if that was a Flite Test model I would have built it by now. It just needs a bit of colour otherwise it will get lost in the grey sky and you could easily lose orientation ?
Something vibrant I would recommend, paricularly if you intend to fly it at any sort of distance or height
Must admit I did the same with my Bushwacker flying it un-painted, only because I was too impatient and wanted see if it would fly. However, I did keep it close in for its maiden, as I could have easily lost orientation that day, as the sky was a greyish white colour and it blended in too well.