D&D General Why do we color-code Dragons?

[Corinnguard]

They might get thrown off by gem dragons or linnorms or other less common stuff but the basics should be mostly well-known.

I am not so sure about some people getting thrown off by the gem dragons since they too are color-coded. The linnorms otoh aren't color-coded, at least the ones in PF1's many bestiaries aren't color-coded. I haven't seen the ones in D&D yet.