Is your 2012 3800 painted? If so, which color scheme? (there were two as I recall. A brown version and a grey version). I have the brown version in my signature picture.
If you don't have a painted rig then the white is just the gelcoat and I don't believe it has a paint code.
I have attached a PDF of a message I received a long time ago from Keystone on the very same questions. The PDF does list the colors but I had no luck getting Sherwin-Williams matching them. Given the age, I assume the paint has faded slightly from its original color. You might be better off getting a paint shop to scan it with the paint color tool and come up with a new formula.
Montana 3700 - Big Sky Paint Codes (2012 & 2013).pdf