I've known my share of set painters and set designers over the years, I'm still friends with some of them and they let me light their work (which must mean something!) They've asked me at various times "How do you know what colour to use in the lights?" and my answer a lot of the time has been "How do you know what colour to paint the scenery?"
If I'm not spattered with paint from a flicked brush of paint the answer mostly is "Fair point". It doesn't help though when a fraught teacher asks "How do I find out how to paint scenery?", imagine my delight that I found this site from a gentleman in Ireland who has an impressive portfolio of images of sets he's painted but importantly - he's happy to share how & why. Have a look and pass the link on - you never know, it might come in handy!