The sentence B contains an error. It should be: The walls or the ceiling is going to be painted beige. The rule is that when there is an "or" in the sentence, the verb is directed by the noun closer to it.
When talking about things that we did in the past but don't do now we can use the expression used to. The negative form, to talk about things which we didn't do in the past but do now, is didn't use to.
Used to and didn't use to are only used to talk about past habits. There is no present form. To talk about present habits we can use the present simple. When using these structures there are often two parts to the sentence, something about the past and something about the present.
