When aesthetic braces are first fitted, they look clean – white (ceramic) or clear (sapphire) – and as invisible as possible. Most often, when a patient wants their orthodontic treatment to look as discreet as possible, they choose clear elastics to hold the archwire in the brackets. The archwires can be white too, again for appearance. At the end of the appointment at which the braces are fitted, the patient leaves the clinic looking their best. Over the following days and weeks, depending on their habits (smoking, coloured drinks) and diet, the look starts to fade. The clear elastics start to turn yellow, and the white archwires gradually lose their coating and look more and more metallic. By the next orthodontic appointment, in some patients the aesthetic braces don't look quite so aesthetic. At the check-up the elastics and archwires are changed, and the braces' original appearance is restored. The reason aesthetic braces seem to turn yellow is usually the elastics, and changing them fixes the problem. The braces themselves change colour only very slightly, and only over a long period (years) in the mouth.
This answer is for general information. Your own case needs an examination by an orthodontist.