You will keep playing games until you get a UV. Every time it rains it could go green or murky without one, no big deal for the fish but if you want clear water just get the UV.
I've never had any green water problems w/rain. Brown once or twice when heavy rains washed soil/mulch in. Chicagoland has had a really crappy summer so far. Lots of rain lately and not causing green water.
Rains can cause pollutants to wash in from lawns, drives, decks, patios, etc. that may be the cause.