Poly-carbonate Sheet yellows after a few years, but is much 'tougher' than Acrylic. NASCAR windshields are made of Poly, and it would replace glass in all autos if it could be made not to yellow. It's tough stuff. Acrylic should stay white for many years, but will became brittle sooner than Poly. If it is Acrylic, and it's yellow, then it's really old and should be replaced. At the least, advise your client you will not be responsible if breakage occurs. If Acrylic is yellow, you can bet it is brittle. Talk to your sign supplier about Acrylic SG (Sign Grade).