As far as the camera is concerned, light and bright is good - I love colours but try to avoid whites and blacks as these are trickier to expose well.
However, if you usually dress in dark clothes, that's fine as it's essential to look like you in the finished photos.
Also bear in mind the season and dress accordingly - shorts and light tops/dresses in summer and coats and scarves for your autumnal or winter shoots. Cardiff is a particularly exposed city and can be very windy, so bear this in mind. Bristol is more sheltered. Accessories are great as well - think sunglasses, bags, hats, jewellery - even pets.