Just like Reedbeta said, it's easy to make the materials to bright.
I did some research in this when I wrote my bidirectional pathtracer (a fancier kind of raytracer), and it turns out white paper has a reflectivity of somewhere around 50%.
You'd expect that to be a medium grey, but it's not. It's all relative to how bright your lightsources are.
What I ended up doing was to specify all my colors like red = (1, 0.01, 0.01) * standardReflectiity, where "standardReflectivity" is 0.5. Notice that even my "red" has a tiny bit of green and blue, since no real material is 100% black or pure in the color.
This way I can divide the light power by the standard reflectivity to get the same exposure, in case I need to change it.