Try pressing Fn-1. Fn and the number keys will let you cycle through a few color modes.
Fn-minus lets you cycle through some predefined static colors, but iCue doesn't seem to be able to change any of them.
The K68 page advertises the keyboard as having 8MB of internal memory. I'm able to click the "save to device" button in iCue (and even confirmed it's writing to the device by watching Wireshark) -- but I can't seem to actually overwrite any of the default hardware modes.
Lots of people are struggling with this. Is it actually not possible to change the colors?
