These are the different robe colors represented by what type of ritual, ceremony or celebration you are attending.
Blue: Worn by healers
Orange: Worn for Sabbats or protective rites
Red: Also worn to Sabbats and to attune with the fiery God
Brown: Wear to attune with animals
White: Great for meditation. Worn to Full Moon celebration
Yellow: For those involved with divination
Green: Great for those involved with Earth
Black: Symbolizes the night and the universe. Which means the ultimate source of divine energy.
Info from Windy Nightstorm