Use ROYGBIV to make acrostic poems. Color laminating plastic, or clear-vu folders with markers for a see-through effect...

For a yearlong theme, maybe use the pot-'o-gold idea - that something good is found at the end of the rainbow. Each month it could be a different word placed at the end of the rainbow thing. You could use character words (responsibility, caring, etc.), study words (practice makes perfect, etc.), creativity words (dream, imagine, wish, etc.), vocabulary. Use the concept of a rainbow as a bridge from here to there, that if you dream big, plan, word hard, you can reach the end of the rainbow... and your goal.

Another thought would be to use rainbows as a launch for studying: water, condensation, light, clouds, etc.