"at the kitchen" is pretty uncommon in English. I don't think you would ever say "at the kitchen" about a kitchen in your house or another person's house... the only way this makes sense to me is if you're talking about a kitchen somewhere else, like "at the kitchen (that I work in) one wall is red". But without that context it sounds really strange.
I just talked to a friend about this and I think the distinction is that the kitchen in a house is just a part of the house, so you can only be in it, not at it. But if you work at a kitchen, you can refer to the kitchen as a whole, not necessarily as a smaller part of a larger whole. To be "at" something doesn't mean you have to be inside it, you can be "at" a place if you're standing in front of it. But you can only be "in" a room, not "at" one.
In Brazil, it is quite common to use another color for only one wall. At my house, 6 rooms have a different color for one wall.
Could someone explain why "One wall in the kitchen is red" is an incorrect translation? Or, how would you translate my sentence into Portuguese so i can understand the difference?