You can update multiple rectangles each with 0ms delay. Issue is that most browsers don't like dealing with 0ms updates, and will consider rectangle update a frame update. Some browsers will silently increase delays. 
You can pause once all necessary rectangles for frame have been updated. 
You can have full colour image provided that it is built from multiple rectangle updates, where each rectangle contains its own colour map. 
 : Browsers not respecting GIF rectangle update delays : http://nullsleep.tumblr.com/post/16524517190/animated-gif-mi...
 : gifsicle [image] --delay [delaytime] #[start]-[end] : http://linux.die.net/man/1/gifsicle
 : True Colour GIF : http://phil.ipal.org/tc.html