Learn Polish: Moja mała siostra lubi różowy kolor. - My little sister likes the colour pink.

They are describing siostra (sister), which is a feminine noun. In Polish, adjectives and possessive words must match the gender of the noun they describe. Feminine dictionary forms almost always end in "-a".

I learned that "I like" is lubię. Why is it lubi here?

Lubi is the "he/she/it" form of the verb. Because the subject of the sentence is your sister ("she"), you need to use lubi instead of the "I" form (lubię).

Why do we say różowy kolor (pink color) instead of just różowy?

In English, we usually just say "I like pink." In Polish, it sounds much more natural to include the word kolor when talking about liking a specific color, making it literally "likes the pink color."

Doesn't the verb lubić (to like) require the accusative case? Why didn't różowy kolor change its ending?

You are right that lubić takes the accusative case (the form used for the direct object of a sentence). However, kolor is a masculine "inanimate" noun (a non-living thing). For these types of nouns, the accusative form looks exactly the same as the basic dictionary form, so różowy kolor doesn't change!