I think I understand Graduated Neutral Density Filters, however, what are the purpose and how do you use Graduated Color Filters? (IE Red, Yellow, Blue, etc)
Are they worth it?
I've got a graduated (orange) "sunset" filter I bought on impulse and used once. In the right circumstance it can enhance a sunset, but to be honest, nothing you can't do in post processing.
If doing B&W photography, a graduated red filter could be used to darken the sky and make the clouds more dramatic (using the red half of the filter for the sky, as it will darken blues). The advantage here is you can affect the sky and clouds, but not have an impact on the foreground.
I would say they are largely a novelty item. They'd have their use in specific circumstances - so if you have tons of gear and $$ an happy to carry them around until they're needed, sure. Otherwise not the most useful item in the bag.