While animating colour may not be possible yet, the approach I used with Terratweak/TG0.9 (
http://www.path.unimelb.edu.au/~bernardk/terragen/terratweak/) would work with TG2 as well and does have a number of positives in terms of controlling the colour changes.
Rather than try to animate a colour, I create two surfaces as a start and endpoint for the sequence and then animate the distribution controls to produce the transition.
This allows you to have multiple transition effects with far more variation and control over the end result...
- colour 2 spreads across colour 1 (e.g. high fractal breakup, low -> high coverage)
- colour 1 fades into colour 2, with/without changes in fractal breakup
- altitude/slope fades
- individual fading of child surfaces at different rates
- changes in colour/reflectivity,luminosty etc.... functions
- combinations of all of the above
The animation linked above did not include any animating of colours. I'm really looking forward to doing this sort of stuff in TG2. The trickiest effect I can think of so far is making leaves disappear off trees in a "realistic" fashion.
The other potential problem with animating colour is that you run the risk of getting noticeable jumps in colour between frames, just as you can get banding in gradated fills. This can occur if only one of the RGB channels has too small a range of variation and tracking down the fault by looking at the rendered result can be very difficult.