Many Wiccans and Pagans prefer to perform ceremonies and rituals in special robes. If you're part of a coven or group, your robe might have to be a certain color or style. In some traditions, the color of the robe indicates the level of training a practitioner has. For many people, donning the ritual robe is a way of separating themselves from the mundane business of everyday life -- it's a way of stepping into the ritual mindset, of walking from the mundane world into the magical world. Most people prefer to wear nothing at all under their ritual robe, but do what is comfortable for you.
It's not uncommon to have robes for the different seasons, symbolizing the turning Wheel of the Year. You can make one in blue for spring, green for summer, brown for fall, and white for winter -- or any other colors that symbolize the seasons for you. Do take the time to put some thought into your color selection -- it used to be that most Wiccans wore white robes, but many people prefer to use earth tones, because it's a way of establishing one's connection with nature. Some people choose to avoid black, because it sometimes has negative connotations, but use the color that feels right for you.