Can you put a lighter shade composite on top of a darker composite?

Composite bonds well to enamel, but not older composite.  Layering composite over older composite usually does not work or results in a very thick restoration.  You are likely better off shaping the tooth for a porcelain veneer.Do you have a composite VENEER, or a composite FILLING?  Horror stories of the temp or pain is usually associated with clinics that have little skill or experience, as we rarely see cases where a veneer fell off or hurts.  If you see the right dentist, it should be uneventful, minimal to zero pain, look great and last a very long time.

Any horror stories about veneers are probably exaggerated. Only .5mm of enamel is removed. Seek out a dentist who uses the Cerec AC system. This will produce the veneer in less than an hour if it is just one tooth. That means you won't have to worry about a temporary! To answer your question, yes, composite can be added over it, but I don't think you would like the result. Composite doesn't last for more than a few years until it begins to darken  and chip. Porcelain is the best choice for cosmetic work.