So what are you supposed to wear when you cast your circle? Good question! It depends on what tradition you want to follow, what your personal opinions are about circle clothing and whether your circle is for an Esbat or a Sabbat.
Traditionally Alexandrians and Gardnerians often go skyclad in circles. This means they are "clad in nothing but the sky" so they're nude! That might be OK if you don't mind other folks seeing you in your birthday suit and if you feel safe doing so. However, you won't find me doing rituals wearing nothing but the skin the Goddess gave me when it's bitterly cold as it can be in an Adelaide Winter or damn near roasting as it often is during an Adelaide Summer! Even more, I'm not about to be eaten alive by mosquitoes either, so for me, it's robes all the way!
Personally I love the colour green and the traditional colours of Oak and Mistletoe are green and purple so that's what I choose to wear. But, if you follow the Alexandrian or Gardnerian traditions, you might wear robes relevant to the degree you've attained. As a Universal Eclectic Wiccan, my colour choice is personal. I believe it should be for you too.
In terms of the material you make your robes from, it should be a plain colour and made from a natural fibre, perhaps wool for cool climates and cotton for warmer locations. The problem with cotton of course is that you have to iron it! So think about the workload you might take on with the material you use. These days, many Wiccans make their robes from penne velvet because it looks nice, is very easy to throw in the washing machine and wash and never needs ironing. Alternatively, you could have lightweight robes for summer and a nice woollen cloak to go over your robes during the winter months. Patterns for robes are easily available on the internet and essentially they are just a large T shape with the top cross bar for the sleeves. Servants of the Light have a great web page you can access with both a simple and tailored pattern at http://www.servantsofthelight.org/knowledge/makingrobes.html. These very simple patterns are easy to cut out, even easier to sew together and can be decorated with any special embroidery that you feel suits your needs. Alternatively, you can always buy a special dress or set of clothing that you save only for ritual work. Don’t forget the cords (belt) as well that you can tie round your waist. Your cords can be a length of plaited wools or cotton ropes and from those cords you can hang a small bag to hold anything you might need in circle and your athame sheath.
Popular contemporary practice encourages Wiccans to wear their robes at both Esbats (Full Moons) and at Sabbats (The festivals in the Wheel of the Year). Whatever colour or style of robes you choose to wear, they should be kept only for ritual work. They should be the special clothes you put on just to honour the Gods and Goddesses. They should be kept clean and well maintained so that you can feel proud to wear them before the Divine.
Smiles and blessings, Amethyst