Update color-definition.md

[LordPachelbel]

rgba() can do a lot more than what is documented, which I only just today discovered when, out of curiosity, I tested whether it could convert RGBA hex notation to RGBA.

[matthew-dean]

@LordPachelbel Just to explain, there are two behaviors you're discovering here.

1. #RRGGBBAA notation is supported, by itself
2. It's not that rgba has a secret way to convert #RRGGBBAA, it's that if you pass a color as the first param, it will return said color. That's to support rgba([color], [opacity]) notation, but opacity is optional (hence the behavior you're seeing). hsla() has the same behavior.

So it's basically:

1. Color in -> color out for rgba and hsla (as well as rgb and hsl ?)
2. #RRGGBBAA is a color

So the documentation may want to reflect that.

[Baro1905]

LordPachelbel:patch-1