The colour of a tooth isn't the same across its whole surface. If you look closely, you'll notice that a tooth is whitest in its central part, darkest near the gum, and in some people slightly greyish at the biting edge or tip. This is down to the colour and thickness of the enamel. Where the tooth looks more yellow, the enamel is thinner and the inner layer of the tooth – the dentine – shows through; at the biting edge there is mainly enamel, which is semi-translucent, and this gives it that slight grey tinge. Braces are generally placed in the middle of the tooth and cover its whitest part. They also cast a shadow and darken the rest of the visible crown. Even white (ceramic) braces, precisely because they are so white, emphasise the yellowness of the tooth near its neck (by the gum). This is the most common reason teeth look darker with braces. Don't worry – it means that on the day your braces come off, your teeth will immediately look whiter.
This answer is for general information. Your own case needs an examination by an orthodontist.