Many things can darken or lighten a finish. In this case it is most likely UV, but could be a reaction to the grill cloth and/or the grill frame from the off gassing of the finish. Common catalyzed lacquers (the most popular finish) can take over a month to cross link and fully cure. Prior to that formaldehyde amongst other things are emitted that reacts to the finish or veneer. I'd try putting it in direct light for a few weeks. A lot depends upon how much UV blocker is added to the finish by the finish manufacturer.
JohnK,
I know you are a fan of shellac and the process called french polishing, however, shellac is a plastic too. Furthermore, anything beyond the first coat of finish or common penetrating stain is all of the penetration you will ever get. Once it is sealed, you maybe able to melt into the finish, but not through it.
Wiki:
Shellac is a natural polymer and is chemically similar to synthetic polymers, and thus can be considered a natural form of plastic. It can be turned into a moulding compound when mixed with wood flour and moulded under heat and pressure methods, so it can also be classified as thermoplastic.
JohnK,
I know you are a fan of shellac and the process called french polishing, however, shellac is a plastic too. Furthermore, anything beyond the first coat of finish or common penetrating stain is all of the penetration you will ever get. Once it is sealed, you maybe able to melt into the finish, but not through it.
Wiki:
Shellac is a natural polymer and is chemically similar to synthetic polymers, and thus can be considered a natural form of plastic. It can be turned into a moulding compound when mixed with wood flour and moulded under heat and pressure methods, so it can also be classified as thermoplastic.