Apologies for the late reply ... I've been swamped with work lately ..
I suppose for a nice shimmering effect, one would like to use multi-tasking - something the Arduino is not designed for (with some tricks task-switching may be possible, but it will not be easy).
An idea could be to have a random LED "grow" to a certain color and the "decay" again to its original color (as defined in you palette).
This would require a few parameters;
- The affected LED(s)
- The target color
- The number of growing steps
- The number of decaying steps.
- The current step (maximum = growing steps + decaying steps, so we know where we are in the process.
Based on the steps and the original color, we could calculate the "new" color.
If the current step is less or equal to the number of growing steps, change the color towards the target color (where the number of growing steps = 100%), and if the number is larger than the number of growing steps we decay the color back to its original color.
Sounds like a real challenge ... and honestly, since I'm running very low on time, it would be hard for me to build a hardware setup and experiment with this cool idea.