Most brass nipples are chromed so they look silverish. Aluminum nipples tend to look a bit duller than the chromed brass.
If you want an LX rear hub, they make LX labelled dynohubs as well so they can match; might have to source it from a European site though, I think it's part of shimano's LX trekking group that isn't popular or sold in North America from what I can tell. There's a set of wheels called the
handspun pavement series 5 that comes with a mavic a319 rim and an LX dynamo on the front but it's only 32 hole, which might be a problem if you carry heavy front loads. The mavic a319 isn't a bad rim, double eyelets and all that jazz...
here's the rear wheel to match I think it's cheaper than buying all the parts at retail prices and you can get a wheel hand tensioned pretty cheaply.
I myself had good luck so far with sun cr-18 rims, the pinned rim joints were all smooth after buildup. I dunno if they come double eyeleted or not, they do usually have single eyelets. T they are a great value IMHO and since rims are a kind of consumable I'm not really on board with the $100+ rims that are becoming a thing these days... a crazy strong rim isn't gonna make up for a mediocre wheelbuild but a good wheelbuild will make a basic rim into part of a strong wheel... probably tensioned spokes do all the heavy lifting. I've never had to replace a spoke on the side of road, so I'm lucky like that I guess... never had any crashes to damage a wheel is part of that too.
Another thing if you are really considering 40 hole rims, the mavic a719 does come in a 40 hole version... and you could use a shimano tandem hub in the rear.