A function like mergeLeds(byte firstLed, byte lastLed) could calculate the crossover colors for LED1 to LED6 if the first and last led are given to this function (LED0 and LED7). If you "move" LED6 to the left or right and let the function calculate the new leds in between, this could look nice, too smiley
yeah, morphing colors - can be done easily with linear interpolation with the map() function 
with respect to the double array's Think you should make one without the high low arrays and keep the RGB array intact.
I expect that just in time merging is fast enough in practice.
It would make the lib simpler too as you can drop the update() functions.
OTher ideas:
- knightRider sketch
- test() to test complete led array R 0..255 G 0..255 B0..255 ==> better make that a testsketch.pde
- binary counter sketch. 32 leds can just represent the 32 bits in a long...
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