Scripted G-keys must also have an associated trigger event, so enclose the cycler with:
if (event=="G_PRESSED" and arg==6) then
--Colour cycler goes in here!
end
Also, trigger events can't be used for loop arguments, so you can't kill a loop with G-key press/release or profile activation/de-activation events, but you can read the M-key state (NOT M-press/release events, but the currently active M state) and use that to kill the loop. At least, that's my experience.
Unfortunately, if the loop is running when the profile changes, the profiler attempts to abort the script, and borks instead. It's waiting for the script to end, but the script is stuck in a loop, and the M keys become unresponsive.
A way to get around this is to include a break sequence (like left-Alt + right-Alt) to abort the script. This should still abort the script even when the profiler is frozen. It works for me, anyway. 
Try replacing the line:
until ( event=="PROFILE_DEACTIVATED" )
With (for example):
until GetMKeyState()=3 or (IsModifierPressed("lalt") and IsModifierPressed("ralt"))
Then just press M3 or both Alt keys (or whatever the Mac equivalent is) together to stop it cycling - only the dual Alt press will be functional if the profiler is frozen.
The code should end up like this:
function OnEvent(event, arg)
if (event=="G_PRESSED" and arg==6) then
local r = 0
local b = 255
rdir = 1
bdir = 1
repeat
r = RedCycle(r);
b = BlueCycle(b);
SetBacklightColor(r, 0, b);
until GetMKeyState()=3 or (IsModifierPressed("lalt") and IsModifierPressed("ralt"))
SetBacklightColor(129, 129); --white
end
end
function RedCycle(r)
if (r >= 255) then
rdir = 0
r = 254
Sleep(150);
return r
elseif (r <= 0) then
rdir = 1
r = 1
Sleep(100);
return r
elseif (r < 255) then
if (rdir == 1) then
r = r + 1
Sleep(15);
return r
elseif (rdir == 0) then
r = r - 1
Sleep(45);
return r
end
end
end
function BlueCycle(b)
if (b >= 255) then
bdir = 0
b = 254
Sleep(100);
return b
elseif (b <= 0) then
bdir = 1
b = 1
Sleep(100);
return b
elseif (b < 255) then
if (bdir == 1) then
b = b + 1
Sleep(55);
return b
elseif (bdir == 0) then
b = b - 1
Sleep(15);
return b
end
end
end ...but I haven't tested it, so I may have borked it. You get the idea though, I hope. 
I'm glad it works - the credit goes to Davs, of course, as I only added a couple of lines for you.
I honestly don't know LUA. There are some truly brilliant scripting examples all over these forums (and elsewhere) that are great to learn from.
I just got to grips with the G-series implementation by diving in - it's surprisingly easy to get along with, and I quickly learned the limitations while trying to do stuff that can't be done. Bit by bit, I'm starting to learn what can be done.
I created a colour cycling script for the G19 when I started. It's not as elegant as Davs, or as involved as KGober's HSL-based fader, but it was a good way to learn. You should think about what you want, and just jump in and try it - I bet you'll find it a lot easier than you think.
