The plan worked fairly well for the Trafalgar Blue sample - I added a three quid pot onto an order I was placing anyway with one of the Moggy suppliers, and it thinned down and sprayed without difficulty, apart from some blooming caused by cold damp conditions and my attempts to speed the drying process by blowing compressed air at it.
Royale Blue was a different story. I could only find one place that sells touch-up pots of it off the shelf, and with the postage and everything it worked out a bit expensive. Not only that, but the paint turned out to be nearly as thick as margarine, and it wouldn't mix with cellulose thinners so I couldn't spray it. I don't know what type of paint it is because it didn't say on the advert and it doesn't say on the pot itself, but I'm pretty sure it isn't cellulose. I tried to brush it on, but it was too thick for a brush even, and I wound up using a scraper to spread it onto the test-piece like buttering a slice of bread. Needless to say the result is very bumpy.
Getting accurate photographs of the two samples proved almost impossible, plus the Royale hasn't fully dried yet and I need to polish the bloom off the Trafalgar, but here's the most accurate photo I managed to get tonight:

Royale on the left, Trafalgar on the right.
My first impression is that Trafalgar Blue is darker than I expected (it's darker than it seems in the photo) and Royale Blue is lighter than I expected (the photo looks about right on my monitor), but it seems to me that the Royale is a slightly "deeper" blue than Trafalgar if you see what I mean. Not darker in shade, but stronger in hue.
I'm going to flat and polish the samples tomorrow, and I'll try to take a better photo of the two afterwards and post it.