I want to know the best way to colour these models–I assume that I need to prepare the card/paper with some sort of spray–what?
Not necessarily, it depends on what paints you used. Clear acrylic spray does give a good surface to work on, and also has the added advantage of keeping the card from absorbing atmospheric water vapor over time. I sprayed a model boat I made with Testor’s Dullcote following assembly, and before adding touch-up paint. That gave the entire hull a uniform level of reflectivity, which was a real plus as the kit came with two different types of papers with different sheens. (This was a pre-colored kit, so I did not need to do major painting.) For my money, spray the model with clear flat acrylic.
