Having finished the second test model I feel it was a great success.
1. The blue looks loads better, the brighter highlight means it looks like a much richer deeper blue and definitely makes the highlights pop a bit more. I'll be keeping this method for doing the blue.
2. The silver is more obvious from a distance now that it is brighter. This makes him look more knightly and less like an ultramarine, Double win!
3. I didn't try the yellow for the shoulder pads. I couldn't bring myself to do it for a couple of reasons, firstly blue and yellow armour with red boltguns would have been too many primary colours, secondly white says 'astral' to me and yellow says 'solar' (starlight is white, sunlight is yellow). The white looked much better when it was thinned base paint rather than layer paint so I am happy to stick with it. I'll just have to tolerate the occasional ultramarine confusion.
I'm going to roll out this scheme on three more models to check it batch paints ok and then I am going to have to start thinking about basing and conversions to make the army stand out from just a custom paint job.
Any ideas for either are very welcome.