I think I know what causes the default color to be returned: After Aero Adjuster opens the wallpaper file to sample its color it forgets to release the file again. Thus Aero Adjuster attempts to open that already opened file when the wallpaper cycle is repeated and the default color is returned. Could you please check whether Aero Adjuster behaves like this on your PC?
That shouldn't happen and yet it happens and it's absolutely reproducible here. It actually surprises me that this error has gone so far without being reported. Thanks for doing so, I'll try to fix it. You'll need version 4.0 (the full one, not the "Client Profile" or whatever one) of .NET Framework to be able to run Aero Adjuster 2 btw (which should be finished around next week).