Check out the documentary "Trinity and Beyond" - it has a lot of quality footage, much of it colour.
There's ten minutes on youtube here - https://www.youtube.com/watch?v=JvnWXf6UZXY. Captures the essence of the film nicely. There's this weird 'end of the world' kind of undertone to the film. Genuinely scary stuff.
This is some absolutely incredible footage. I had no idea there was color footage of the actual bombings in Japan.