Learn Italian: Coloro che boicottano questa marca difendono la natura. - Those who boycott this brand defend nature.

Both translate to "those who," but coloro che is more formal, often used in writing, journalism, or formal statements. Quelli che means exactly the same thing but is much more common in everyday spoken Italian. Since this sentence sounds like an ethical or political statement, the elevated tone of coloro che fits perfectly.

Does coloro change based on gender, like quelli and quelle do?

No, coloro is invariable for gender, meaning you use it for an all-male group, an all-female group, or a mixed group. It is, however, strictly plural. If you needed the singular "the one who," you would use colui che (masculine) or colei che (feminine).

Could I use the subjunctive and say Coloro che boicottino instead of the indicative boicottano?

You could, but it changes the nuance. Using the indicative boicottano treats the boycotters as a real, currently existing group of people. If you used the subjunctive boicottino, it would make the group hypothetical, meaning "Anyone who might boycott..." Since this sentence states a fact about an existing group's actions, the indicative is preferred.

Why is there an article before natura (la natura) when English just says "nature"?

In Italian, generalized abstract concepts and broad categories require a definite article. Whenever you talk about "nature," "love," or "society" as a whole concept rather than a specific instance, you must include the article.