Why is my Invisalign tray greenish-yellow?

Thank you for posting your question.  An accuarate assesment can only be given if it is seen.  Show your dentist what your are talking about and see what he says.  I can only speculate...You may not have cleaned the tray thoroughly enough. There may be something growing on the tray or it could be residual plaque build-up with staining.  You should use a toothbrush to clean the tray and remove any build-up that may be there. Placing the tray in an 8 ounce glass of water and a few drops of bleach may help to kill anything that may be growing on it. Rinse the tray well before placing it back in your mouth.  Hope that helps!  Good Luck!