Codominance occurs when two different alleles for a trait are both expressed in the phenotype of an organism. In the examples you provided, the correct example of codominance is:
Black and brown spotted coat color in horses.
In this case, both coat colors (black and brown) are fully expressed, leading to a spotted appearance. The other examples do not represent codominance. For instance, black and yellow coat color in Labrador retrievers is an example of complete dominance, while tall and short plant heights would also show a dominant trait and not codominance. Seed shape in pea plants typically displays complete dominance as well.