If a funeral announcement describes the funeral as "a celebration of X's life", what would you wear? It will be held at a crematorium and there are no other pointers, such as a request to wear something in the deceased's favourite colour. I'm guessing that a black suit would be too much, but I'm not sure which colours (or level of formality) would be appropriate.
The deceased was a teenager, so despite the focus on his full life and many friends, it's obviously going to be a very difficult day.