Answer:
The walls or the ceiling are going to be painted beige.
Step-by-step explanation:
Subject-verb agreement refers to the correspondence of a singular subject with a singular verb, or a plural subject with a plural verb. Thus, the correct version is the following:
The walls or the ceiling is going to be painted beige.
The verb must be singular (is) because its nearest subject is also singular (the ceiling). In fact, when in the subject two nouns are linked by or, neither or either, the verb must be singular if they are both singular, and plural if they are both plural. When one of the nouns is singular and the other is plural, the verb must agree in number with the closest one.