I am still trying to fix that problem. Screen Tone changes are in the core script. If you look closely, you see they kind of work, but not in the way they are supposed to. So, what do ya'll think of the Regional Plug-In? Seasonal requires the Game_Time class from 7.0, but it needs to be edited a bit before being used.
About mixing weather, that's not exactly my field of expertise. I suppose I could try and edit Game_Screen to allow a max of two or three effects at a time, so when you start a weather effect manually it will add that effect to the first empty weather slot unless you pass a third argument that declares a specific index to be used. May sound confusing, it'll be easier to explain when I do it.
Alright, I'll try that. I am not guaranteeing a perfect release like my Inners Plug-In (which is so awesome, and simple!), but I'll do my best to complete it.
A thought: another way to do this would be to alter RPG::Weather like Ccoa did, but from what I've seen in her script, it would probably double its size.