The elastic ties on conventional braces are made of silicone. This material can absorb the colourings we take in with our food. Spices such as turmeric, curry and paprika, and the foods and drinks that contain them, turn clear elastic ties bright yellow. The lighter the colour of the elastic ties you've chosen, the more likely they are to change colour. Blue ties combined with yellow food turn green, and pink ones turn orange. Green, yellow, dark blue and all dark, deep-coloured elastic ties don't change noticeably.
This answer is for general information. Your own case needs an examination by an orthodontist.