The English translation is awkward. In the U.S. we would say, "You never wear the brown shirt."
Poner is used as a reflexive verb here, so you need to use the respective object pronoun. You "put something on yourself", so to say. You can also say "Tú nunca te pones..."
Does poner always require an object, which is why this is "te pones"? I guess we omit that in English. "You never put the brown shirt on (yourself)".
The shirt is an object in this sentence, though. But yes, that's approximately the reason here. Poner usually needs another argument, you put something (DO) into some place/condition/state (IO). English uses reflexive verbs much less often and mostly opts for using intransitive verbs (or in this case, verbs with a reduced object count).
Why is it "pones" and not "ponerse" in Nunca te pones la camisa marrón? I thought pones means "put" and ponerse "put on"?!
Ponerse does mean “put on” but it is an infinitive verb, so it needs a conjugated verb to precede it. For instance: Ella quiere ponerse la camisa. She wants to put on the shirt. Tu quieres ponerte la camisa. You want to put on the shirt.
In the sentence given, pones is the conjugated verb, and to make it reflexive, the te is needed in the sentence. But you can’t add the te to the end of a conjugated verb; it needs to go before the verb.
You cannot attach object pronouns to the end of a conjugated verb (unless it's conjugated in imperative, the command form). The only other verb forms you can attach the pronoun to are the infinitive form ("No puedes ponerte esa camisa") and the gerundio ("Saliste de la habitación poniéndote tu camisa").
Ponerse is "to put on", i.e. you're in the act of dressing yourself. With "wear" (llevar, usar) you're already done with dressing.
¿por qué debo poner en la camisa marrón cuándo tengo ... GRIS, o ... VERDE o ... ROJO?
Is "cafe" and "marron" (sorry I can't do the accents) the same, or is it different shades of brown?
Both words just mean "brown" in most circumstances. They are usually used synonymously, with café more popular in LatAm. If the circumstances are there to talk about specific shades, marrón means "chestnut" and café means "coffee"