for id in spheres:
radius = get_radius(id)
if radius >= divide :
Edit: I misread the question. If you want to find and colorize the largest only (and all the others different color) . You can color them all the smaller color and through @Dancergraham s sorting solution grab the largest and color that different.
Even better: largest = mylist.pop() removes the final, largest item in the list and leaves all the rest so you can iterate through them. I like this solution because Python lists are optimised for quickly adding and removing items at the end.