Can you use watercolor pencils on cardboard?

Sure you can. Whether the colors will actually be vivid enough to show, I don’t know. Try it on another piece of cardboard if you don’t want to try it on the box.

I agree with testing first. Also remember that with colored pencils you may have to put down several layers of color to get the depth you want, and burnish them, especially if the surface is rough or uneven.

Yes you can.