Awesome.
I guess it's as useful as palette cycling - simple in itself, but great for adding a bit of interest. Especially as you allow synthesis functions as input.

Does it work just with the RGB colour space, or others too?

Have you thought about doing any other colour space transformations? No idea what use it'd be, but could be interesting!

Those look like they could make very good procedurally generated spell effects; the bottom right one in particular. If you can generate it animated, cut and instead of gradiating red-to-blue, do red-to-alpha, apply a mask over it to make it smoothly non-square, then you could have a very wickedly awesome lightning bolt/ball where every spell cast is different.

And the bottom-left could be poisonous gas, if you do fade-to-alpha. Animated fire could come of this also.

I agree with Servant here any one of those would be great for procedurally generated particle effects. Not just spells but even nebulae in space and such especially if you can animate it in some way.

I like the upper left one because it seems to use more of the spectrum than the others ;) I also think these could be used in some kind of forcefield or magical barrier rendering. Also agreeing with the above, if they can be animated it could lead to some really impressive effects.

That's the cool thing about how I'm building the library. Any of those effects can be animated in many different ways. Flexibility is my main goal, and pretty much every day I'm making new tweaks to how things work. Still trying to catch up on the docs, though. I [url="http://accidentalnoise.sourceforge.net/index.html"]recycled[/url] the old page I used to have up here on gamedev. I'm not much of a web guy, but it doesn't matter, I just need to get something usable. Working on the examples section today.