Adult Yellow (Picric) Dragon

The Yellow (aka Picric) Dragon is a variant of the Black Dragon that has developed different coloration and a different, highly toxic and potentially explosive acid for its breath due to different environmental conditions (e.g. dwelling in a swamp high in nitrates, mineral salts, and various phenol precursors but low in iron, copper, zinc, or lead, even in solution). Other than the coloration, the main thing that is different about the Yellow (Picric) Dragon is that it breathes a toxic cloud of picric acid. This does less acid damage than a Black Dragon's breath (8d8 instead of 12d8), but it has two other dangerous effects as well:

First, picric acid is highly toxic as well as acidic, so anyone caught in the cloud of acid must also save vs poison (DC 18 constitution save) or be poisoned and suffer a level of exhaustion. (Victims must save every round, with each failure being another level of exhaustion. A successful save or lesser restoration or similar anti-poison spell ends the condition and the progression of the exhaustion, but rest or a greater restoration or heal spell or similar healing is needed to end the accrued exhaustion effects.)

Second, picric acid reacts VERY strongly with iron, steel, copper, zinc, silver, and lead, forming highly explosive acidic salts on the surface of such metals. Nonmagical metal exposed to the cloud of acid will become pitted and damaged to the point of uselessness within 1 round (i.e. by the dragon's next turn) unless immediately doused with a good amount of water. Lesser magical items with exposed iron, steel, lead, zinc, silver or copper surfaces will begin to corrode within one minute if not cleaned off (water is the best way to clean), with items of +3 or equivalent (or better) quality resisting the corrosive effect. (+1 and equivalent items have a 10% cumulative chance of corroding for every minute left exposed, checked at the end of each minute, +2 or equivalent items have a 10% cumulative chance per hour left exposed.)

Even worse than the corrosive effects, however, is the fact that the acidic, metallic salts formed when the cloud of acid touches metal are HIGHLY explosive, reacting violently to shock (even fairly light touch) or fire. Affected objects will explode if struck with any kind of force (including Magic Missile spells, which is a speciality of this kind of dragon) or exposed to even a match's fire. The force of the explosion is proportional to the amount of picric salts formed: a very small item (such as a metal arrowhead) will explode when struck for 1d6 damage (per item), 5' radius, DC 18 dex save for half damage; a small item (such as a dagger or short sword) will explode for 3d6 damage, 10' radius (same save); a medium sized item such as a longsword, helmet, greatsword, greataxe, etc will explode for 6d6 damage, 20' radius (same save); a larger item, such as a breastplate, chain shirt or chainmail, plate mail, medium or large metal shield, etc, will explode for 10d6 damage, 30' radius (same save, except if you are wearing the armor, you save at disadvantage); quite large items (a steel door or wall, maybe) will explode for at least 12d6 damage, at least 30' radius, with larger amounts of damage and large radius explosions being possible for very large surfaces. Note that being within the area of effect of one explosion WILL detonate any other affected items, potentially setting off an absolutely devastating chain reaction. (e.g.longsword is dropped, explodes, sets off steel greaves which set off a shield and helmet and plate mail, and so on). 

Yellow (Picric) dragons have several favorite combinations of attacks: breathe a cloud of toxic acid and then dive under water or fly up out of explosion range, wait a round or two for the acids to do their work, and then magic missile a fat target to set off a chain reaction, if possible. They are absolutely devastating against massed troops in armor (or against weapons depots or armories or forges). For obvious reasons, they do not like to fight underwater or in even a moderate rain, though they can breathe water and often lair underwater.

Note that if one has one's wits about them, one can discern that an item being carried has the explosive salts on it and so can attempt to throw it away (turning it into a grenade). It takes a steady hand to do so, however, and a DC 18 sleight of hand check to toss it (or set it down) without setting it off. Armor can't be taken off without detonating it, except with extreme slowness (the same DC 18 save each round for three straight rounds to take the armor off safely--and then a fourth save to toss it or lay it down safely).

The key to surviving this dangerous creature is water, lots of water--or to go after it without any exposed metal other than gold, platinum, or adamantium. (It will react with silver, but not gold.) The acid mixes readily with water and loses its acidity and toxicity quickly; the explosive acidic salts also dilute quickly in water and so lose their explosiveness as well as their ability to corrode, but it takes a fair amount of water to fully cleanse something--at least enough to fully immerse the item and shake it around a bit or to do a serious hose-down. If one is able to dive underwater before being hit by the acid cloud, then one will suffer no effects from the acid cloud

Yellow (Picric) Dragons are quite a rare variant, so there is little lore about them, just a few odd tales of someone thinking they saw a gold dragon, only it breathed an acid cloud instead of fire, and then everything started to explode as people ran around choking. Rumor has it that, if it somehow swallows a significant amount of metal, residual acids in its throat will react with it, forming a potential explosive on the inside--if somehow a shock or fire could trigger it, the results could be devastating to the dragon. (Which might be one reason why it lairs underwater.)