In our grove, Emperor is best when there is still some yellow on the peel up until the point that it gets to the fully red stage. If it gets purplish, it will typically be flat. Sweetheart and Florida Hak Ip are also best when there is still some green and can even be picked when they are about half red/green. In contrast, Brewster should NEVER be picked until it has fully changed color to red (if you like tangy) or dark red/purple if you like mainly sweet. Maybe someone from California could comment since it is probably different, but that is the rule of thumb here in Central Florida