Older cars have the code on the strut top, then it moved to the drivers A post - now they're putting them on the door edges, sometimes drivers, sometimes passenger. The 4 letter code at the bottom of the tyre pressure sticker is, 100%, the colour code.
To be honest, it's the middle two letters which are most important - EYLC is what's on my 306 but at work, EYL or YL work just as well. Not sure why yours starts with an N - it's almost always an E - the C isn't important at all - it means that there's clearcoat (or lacquer) over the paint. The only thing I can think of is that it MIGHT be a special code for a two-tone car - I've personally never seen a two tone pug from the factory so it's possible I've just never had to mix the paint for one. Seems odd though that a paint company the size of ICI have no trace of it either....
EDIT: Having said that, just found this page -
406 Coupe Owners Club which does confirm your colour code. My best guess would be that the black is EXVC and the silver is EVWC, purely based on the fact that those appear to be the black and silver on 406 coupes....
SECOND EDIT: Is your car actually silver and black, or is it one of the "silver and black" special editions where silver and black is the name, not the colour?