Translation:The letter is yellow and the apples are yellow.
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". :)