Question

why is the blue box always blue? is it always blue? will it look blue even and dark under red light?

Answer 1

Yes it is blue even dark and red light.

Answer 2

A blue box always looks blue because it reflects light of blue color and absorbs the other six lights.

When a blue box is placed in red light it, the box absorbs the red light but there's no blue light to reflect therefore it looks dark in red light.