If you think about it, Forever Young really showcases both sides of BP in one song. That's why I wouldn't really consider it 'Black' or 'Pink', it's both.
Like at first you think 'Yeah, this is a Pink song', with the summer party and tropical vibes, like a song suited for summer. But closer to the end of the song, the sun comes down, and then the instrumental starts getting more hardcore, similar to the 'Black' concepts we all know and love. And then the "WHATABUMBUM WHATABUMBUM" is similar to the onomatopoeia used in BP's previous 'Black' concept songs, like BOOMBAYAH, D4, etc.
Just wanted to share this out with the world.