It's a bit annoying trying to color match various swatches across multiple auras. There's a trick that can be done where you select the color from a previous aura with the dialogue open, then select new weakaura and it'll carry over but it's still a hassle.

Color picking tool in addition to what it currently has (circle+transparency slider) should also have R, G and B fields to input colors manually which overrides the selection in the color circle pick.

The colorpicker is a standard widget by blizzard. Fortunately there are addons that replace the color picker and add various additional features, such as the one you have suggested.