In my games, dragons don't have "colors." They all have the ability to change their color to blend into their surroundings, like certain lizards. In BECM, it meant that dragons could hide like a Thief of equal level, in 3.x Edition they got a racial bonus to Hide checks, and in 5E it's advantage on Stealth.
But most importantly, it meant that the party of heroes couldn't anticipate a dragon's energy types at a glance. They had to do a little bit of homework, investigation, or experimentation to figure out what they were dealing with.