A couple of things came to mind as I read your post
1. Cut the numbers 50 from a sheet cake and cover with flowers or other appropriate items, ie figure piped paint brushes or other tools of his trade.
2. Carve a single layer round cake into a painters pallet. To do this cut out a notch for a thumb hole and use a round cutter to cut a hole to make the other grip in the pallet. Then cover the cake with a light brown fondant and smear bright blobs of icing around the edge to resemble oil paints on a pallet you could then pipe your birthday greeting in the center and figure pipe or make from pastilage or gum paste a paint brush posistion the brush so that it is leaves a paint trail and looks like it was used to paint the birthday greeting, do this by making the tip of the brush look like it was dipped in the same color icing as the greeting and leave a trail of "paint" from the last letter to the tip of the brush.