This is really difficult.
I went to a paint shop today and saw hundreds of blues. These guys make it up for you. Literally colour books full of blues. None matched perfectly, or indeed better than peugeot indigo blue.
The computer scanner there could not detect the colour either. The issue is that the decal is screen printed with ink, so you (and the computer) see the white vinyl "through" the ink. The guy said that this was a frequent problem with vinyl signs too, when folk wanted paint to match.
The one way I found to match it perfectly is to spray indigo blue onto a white background, but not too thickly
All getting a bit silly for something I doubt anyone will ever notice