Any, umm, FREE ones?

Toon Boom Studio has that feature, and it is very useful. What they do is have this bar at the bottom of the spectrum, and you would right click on it to add a color, the adjust the color to the liking, and finally, you could click and drag the slider in the bar so you can adjust the order the the g...

It would be even sweller if they integrated that option into the batch export