In my climate (very hot/humid/but no rain with established larger Plumeria) yellowing leaves during the heat of summer can be indicative of being a little bit too dry.

when a plumeria is done with a leaf it just cuts it off and it goes yellow. As the season wears on, this happens to the lower leaves so more energy is channeled to the tip of the plant and the flowers. It is normal to have a yellow leaf here and there. Early in the season it is left over leaves from last season that are now rejected for new. Later it is a sign of preparing for winter. Unless you have many all over the branch, I would not worry. Leaves that are green and yellow often are signs that photosynthesis is not working correctly because there is not enough magnesium to complete the process. epsom salts will cure this in time. Bill