No, you can't become invisible. You can, however, wear robes that correspond to certain aspects in magic which might help with the related spell casting/ritual creation. You'll need to consecrate/bless the robe(s) before ritual purposes to help it better channel the energy associated with its color.
Also, you don't have to wear a ritual robe when holding rituals, celebrating Sabbats, etc. However, should you choose to wear one I've provided a list below to help you choose the appropriate one.
Robe Colors:
Yellow: Divination
Purple: Worn by those who work with pure divine power, or those who wish to deepen their spiritual awareness of the Goddess and God.
Blue: Healers and those who work with their psychic awareness or for attuning with the Goddess in her oceanic aspect.
Green: Empowers herbalists and magical Ecologists.
Brown: Worn by those who attune with animals or by those who cast spells for them.
White: Purification, pure spirituality, and meditation. It can also be worn during cleansing rituals, Full Moon celebrations, and for attuning with the Goddess.
Red/Orange: Sabbats, protective rituals, and attuning with the God.
Black: Protective, symbolizes the night, blackness of outer space (symbolically), and the ultimate Divine energy.
Sources:
Wicca: A Guide for the Solitary Practitioner. Copyright
1988 By Scott Cunningham. All rights reserved.
Living Wicca: A Further Guide for the Solitary Practitioner.
Copyright 1993 by Scott Cunningham. All rights reserved.
This site offers very limited ritual-wear and does not carry robes.
Unless you are from a specific tradition with specific rules on ritual clothing then you can wear whatever you see fit.
I, for example, don't wear a robe. I practice yoga in my circle and a robe wouldn't be at all practical. I therefore wear a special t-shirt and pair of pants I use as ritual wear.
So think about why you even want a robe and what you'll be doing in it, then pick out a robe.
