Need more information please.
Do you mean the big buildings on a dairy where the milking is done? Or maybe an actual small building on a farm used for milking the family cow or goats? Whatever, including miniatures or models, I would make sure there is a proper floor and easy sanitation. All the creamy pastels and Spring flowers would be precious. If you want rustic instead, it’s wood and plaster and white paint, and a wood or thatched roof. Please let us know how we can answer your question better. Best wishes!
Outside??paint it black & white like a giant Holstein;paint milk cows on it;decorate the outside with containers of petunias;add some older milk cans painted in bright primary colors;add some old shutters around windows/doors painted in bright colors;place an old wheel barrow by entrance planted with bright colored petunias-Inside?? add old farm tools to walls;use chairs on walls painted brightly colored as shelving like this https://www.youtube.com/watch?v=qG6nq3HGdYU more https://www.youtube.com/watch?v=NTSLqXIPJPY&t=46s using crates https://www.youtube.com/watch?v=BUFuk-hc_pM milk cans https://www.youtube.com/watch?v=9VDoQbP1qgw add some nice string lights around inside/outside for ambiance and some music for the cows while milking(if you are using it as milking area)
Share more on what a milk house is?
farm talk for where milking a cow is done.....