Why are there five chromatic dragons and not four?

Question

The five chromatic dragon types are already in the original edition of D&D.

In that same game, player characters had access to four primary kinds of damage: physical blows, fire, cold and lightning. In addition, there are several monsters that have corrosive attacks, for example, slimes such as the black pudding or grey ooze, for acid damage.

All dragon types obviously have physical attacks. Four of the dragon types correspond to the four other damage kinds: red to fire, blue to lightning, white to cold and black to acid. Based on that, it would seem natural to have four chromatic dragon types, not five.

Is there any documented design information why Gygax decided to add a fifth type of dragon in the green dragon (with the weird damage of "Chlorine Gas"—back then, they didn't do poison damage)?

Answer 1

The different colours of chromatic dragon were inspired by more than just damage types

The Slayer's Guide to Dragons (2002) includes a preface from author Gary Gygax which explains, to some extent, his thought processes when designing the original five types of chromatic dragon in OD&D.

The first type of dragon Gygax designed, and used in play, was what would become the typical red dragon; a fire-breathing winged reptile, the model for which was a converted dinosaur:

As a matter of fact, what was probably the first dragon used on the table top was a converted dinosaur model [...] with the addition of cardboard wings, the general form of the fearsome red dragon was visible!

For the sake of variety he then added poison-breathing green dragons, inspired by some legends that described dragons as serpents with poisonous breath as opposed to breathing fire, and some that specifically mentioned green dragons (it's unclear if they are the same legends):

Some 'historical' references spoke of dragons as 'serpents' with poisonous breath. There were mentions of dragons of green colour. Thus, it was a simple matter to add the green dragon that exhaled a cloud of poisonous gas [...]

White dragons were next, apparently because that would complete the triad of red, green, and white dragons found in mahjong tiles, and the cold nature of their breath was chosen to match their colour:

[...] the mahjongg game has three sets of different tiles named dragons - green, red, and white. Having played that game since I was a boy, how could I ignore the white dragon? So what form of breath weapon went with that colour? Snow and cold, of course. [...]

Then blue and black dragons were added, and it seems that this was indeed to represent the extra damage types and add even more variety to the kinds of dragon available for players to fight:

After some contemplation, I added the blue colour, as that could well represent lightning, and there was a spell in the rules covering just such an electrical bolt. Acid breath seemed another reasonable form of attack, black represented that well, and thus the fifth kind of malign race of dragons was born.

[...]

That ended the near-complacency of would-be dragon slayers. No longer could a single set of defences and attacks apply when a dragon was known to be on the loose.

So according to this book, red dragons exist because of the common depiction of the fire breathing dragon in European myth, green dragons exist because of myths about poison-breathing serpents, white dragons exist because of mahjong tiles, and blue and black dragons exist for damage variety.

Answer 2

All five chromatic dragons pre-date D&D by several years.

The previous answer citing Gygax's Slayer's Guide to Dragons pretty much nails it, but I'd like to add some additional historical context.

According to D&D historian Jon Peterson's Playing at the World, all five chromatic dragons as we know them can be traced back to a series of articles written by Gary Gygax for the Diplomacy fanzine Thangorodrim from 1969 to 1970. We can be grateful to the Internet Archive for preserving copies of several issues. A "red dragon" is mentioned in issue 2 (Aug 1, 1969), in clear reference to Smaug, the fire-breathing dragon of The Hobbit. Issue 3 are missing from the archive, but PatW assures us that the arctic white dragon appeared in that issue. Issue 5 describes the acid-spitting black dragon; issue 6 the chlorine-breathing green dragon; issue 8 the electricity-using blue dragon; and issue 9 the mottled dragon (later becoming D&D's purple worm).

In OD&D there was a cloudkill poison spell, but it didn't do hit points damage, since poison at that point in D&D's history was an instant kill effect. Poison was still an instakill in AD&D 1e, and even in AD&D 2e and D&D 3e it generally inflicted some kind of debility, rather than points of damage. In D&D 3.5, the green dragon's breath weapon in is corrosive gas, and counts as acid type. It's D&D 4e (2008) that makes poison a standard damage type like any other.

In other words, the unusual quirk here is not that they included a poison type dragon, but that poison didn't generally deal hit point damage until much later in D&D's development. The green dragon's breath weapon was unusual in this regard, since even in OD&D and AD&D 1e, it did hit point damage on account of using the standard breath weapon mechanics for dragons.

Answer 3

You have the cart before the horse

First there were dragons (0D&D). Gary was heavily involved in this decision.

Then there were damage types (3D&D). Gary was long gone.