Let's Dissect 2e's Dragons, Draconomicon Edition - Yellow Dragon

"HEY! I just got through eating this, at least let me clean the place before you start taking pictures."

  

 “Yellow Dragon” sounds like a drug euphemism.

Although the existence of yellow dragons has long been predicted by sages (based on theories of primary colors), the first specimen was spotted only five or so years ago.

So the actual canon reason people suspected yellow dragons existed was “well, we’ve got ‘em in the other two primary colors!”. Sounds about right.