Is a son's wearing of a brown garment, despite his intention to follow the Sunnah of wearing white, considered disobedience to parents, given that the father would have been angry if his son had not obeyed his command?

You should have obeyed your father by wearing a non-white garment, as he had a valid reason for that, and obeying him in forsaking a recommended act is obligatory in such a situation. Since you obeyed him and wore the garment he commanded you to, you are not disobedient. Be keen to appease him, avoid what angers him, and strive to be dutiful and benevolent towards him; for his dutifulness is among the best acts of obedience, and the father is the middle gate of Paradise.