Translation:The letter is yellow and the apples are yellow.
I love that stuff! I caught onto the "gilt" connection, but not gold.
Not a native, so excuse me if I make any mistakes. But I find to fully understand something it's best to try and explain it to others. So, with that out of the way:
It works the same with about any adjective. It changes depending on what it is describing (neuter, feminine, masculine, plural). AFAIK "rosa" doesn't change, however. "Brevet er rosa og eplene er rosa". :)
Brevet is neuter, that's why the adjective needs to be in neuter form as well. You are confusing the definite form with constructs as in "det gule brevet", which describes a very definite yellow letter (that yellow letter). Just because the noun is definite doesn't mean the adjevtice always has to be in definite form as well.
Actually, there are three. One for masculine/feminine, one for neuter, and one for plural. Most adjectives in norwegian change depending on the grammatical gender they describe. "Mannen er gul (maybe he has yellow fever?), brevet er gult og eplene er gule". It's just one of the things you have to memorize.