Hmm, redoing the core dragons for 7 breaths, 7 colors, 7 metals.
Red: Fire
Yellow: Lightning
Green: Wind (Force)
Aqua: Acid (green-blue scales, yellow-orange breath)
Blue (indigo): Cold
Black: Death (Poison)
White: Light (Psychic? Magic? Laser?)
If you’re going to do 7 varieties of chromatic dragon, why not ROYGBIV? Alternatively, the traditional Red, Green, Blue, Black White works perfectly as the primary colors of additive color theory (swap Red for Magenta, Green for Yellow, and Blue for Cyan if you prefer subtractive), with other colors being cross breeds.
Gold: Fire
Silver: Cold
Copper: Lightning
Quicksilver: Light
Iron: Force (maybe magnetism)
Lead: Poison
Tin: Acid (because that's what's left)
I’d definitely give Iron Fire. The symbolic association is too strong to ignore. Flint and steel starts fires, fire is used to refine and mold iron into tools, weapons, and armor, iron is associated with mars, who is in turn associated with fire. Iron dragons definitely need to breathe and resist fire.
I get what you’re going for with lead being poison, but I don’t think the symbolic association is strong. Yeah, lead is poisonous, but think about what it’s used for. It is incredibly dense and insulates well against any form of radiation, so radiant damage might be appropriate, though personally I would reserve radiant damage for either quicksilver or gold. It also doesn’t transmit vibrations well, so I would go for thunder damage with its “breath weapon” being a deafening roar. Also as the basest alchemical Metal, it’s fitting that it wouldn’t have a true *breath* weapon like the more refined dragons.
Radiant works well for quicksilver, but personally, I’d rather give it to gold, and give poison to quicksilver. Mercury is also highly poisonous, and the idea of a quicksilver dragon spitting actual mercury is pretty cool. Gold is symbolically associated with the sun, so it should either be fire or radiant, and in my opinion Iron has dibs on fire. Maybe you could do one a line of fire and one a cone of fire I suppose, but I kind of like radiant as the perfected form of fire, just as gold is the perfected form of baser metals.