Right. It's the problem of making the columns line up. The first bunch of associations match up nicely and the rest...just kind of get shoehorned in. You saw this with medieval tables of correspondences. "Sun...gold...yellow! Moon...silver...white! That was easy! All right, what about the other planets? Mars...iron...red, because rust and the planet's red! Venus...copper...green, because patina and...uh...leaves are pretty! Saturn...lead...black, because it's the darkest planet! Whew! Jupiter...tin...blue...Jupiter was the god of the sky, right? Mercury...mercury...okay, we need a color. Uh, what do we have left? Orange? Purple?"
Now they get to the dragons. "Red breathes fire, so fire! Green breathes gas, so air! White breathes cold, so cold! Black breathes acid, so...uh...water, 'cause acid is liquid? Blue breathes lightning, so air...but wait, nobody's earth. Uh, do we want the green one to be earth...that doesn't make any sense, air is a gas. Or maybe the black one...nah, acid is a liquid. We're shipping in two days? OK, blue is earth! It lives in the desert! That's my story, and I'm sticking to it! Now did we figure out what the devourer looks like since DiTerlizzi quit? OK. What's our favored class for gnolls? Dang, they took sorcerer, thief, fighter, and barbarian. Wizard's off-brand...How about ranger? Nobody picked that one yet for the other goblinoids, right? Well, I guess they live in the woods..."