Is a person's belief that the color black brings bad luck, based on personal experiences, considered an act of disbelief (kufr) or polytheism (shirk)?

Saying: "I expect the color black to bring me bad luck" is not disbelief (kufr) or polytheism (shirk (definition)). However, such phrases should be avoided. Pessimism (tashā'um) is not considered major shirk unless the pessimistic person believes that what he is pessimistic about acts on its own, independently of Allah. But if a person considers something he sees or hears to be an ill omen, it weakens his reliance on Allah and saps his resolve. This is considered minor shirk because he has relied on a cause that Allah has not made a cause. Thus, anyone who relies on a cause that the Sacred Law has not designated as a cause is committing minor shirk.