With a spectroscope, you can see the differences in "white" light. Sunlight is a continuous spectrum, and artificial lighting attempts to mimic that by mixing at least 5 colours. Electrical displays use RGB, but they're wider than I expected. It would be interesting to compare LED lit and CCFL lit LCD screens.
Previously, I had tried making my own spectroscope using a CD and cardboard tube:
I would like to improve my spectroscope by making the slit width adjustable, maybe by using sharp metal edges and a screw mechanism.