It really depends on exactly what kind of "fiberglass" you want to represent, as there are many colors depending on what resin is used, and part thickness.
Old-school polyester is often "that yellowish color", but depending on a variety of factors, can be dark yellow/brown, dark brown, and greenish yellow or dark green.
SMC (sheet molding compound) that raw Corvette OEM body panels are made from may be light to dark gray, uncoated floor panels are frequently (but not always) "that yellowish color" again, with aftermarket stuff coming in a wide variety of colored gelcoats or tinted resins too.
When epoxy is the resin system (as opposed to polyester), very light green to dark green is most prevalent, sometimes light blue or even colorless, while vinyl-ester resins tend toward the brown scale.
If you want to model a composite part accurately, it's good to research your particular application.