Learn Italian: Mi sono piaciuti i rossetti rossi.

The verb piacere (to like/be pleasing) works differently than in English. You aren't saying "I liked the lipsticks", but rather "The lipsticks were pleasing to me." Because piacere uses essere (to be) as its helper verb in the past tense, you must use sono to match the plural subject, which is the lipsticks.

Why does the past participle piaciuti end in -i here?

Whenever you use essere as the helper verb in the passato prossimo (past tense), the past participle must agree in gender and number with the subject of the sentence. Since the subject here is i rossetti (masculine plural), piaciuto changes its ending to -i.

What role does mi play in this sentence?

Mi is an indirect object pronoun that means "to me". It tells us who the lipsticks were pleasing to.

Is rossetto related to the word rosso?

Yes! Rossetto (lipstick) literally comes from rosso (red) plus the diminutive suffix -etto (small or cute). It is a masculine noun, so it takes the masculine plural ending -i to become i rossetti, and the adjective matches it exactly as rossi.