Well that worked better than I could have hoped!
The enamel paints arrived in the week and my wife is out this evening, so while I was keeping an eye on the boys I decided to do some colour tests. That went well and I had a go at painting by hand. I'm absolutely not skilled enough for it, but then I tried masking with sellotape and that worked surprisingly well, I was expecting the paint to seep badly, but it was very minimal.
I very carefully masked for painting the light blue and the red and did those first. I found it was best to let the paint set for about 15-20 minutes before removing the masking (I didn't want to leave it too long and lift the paint with the tape).
I then allowed an hour for drying, and then masked for the dark blue and painted that, another 15-20 mins for the masking and then another hour to harden. I then very carefully cleaned up the edges with a sharp knife before applying the black paint.
For a hand-made effort I'm very happy with the end result. With the macro shot below you can see it's not perfect, but with the naked eye it looks perfect.
Great success!