Learn German: Gefällt euch die Farbe? - Do you all like the color?

With the verb gefallen, the grammar works backwards compared to English. The thing being liked (die Farbe) is the subject of the sentence, not the person liking it. Because die Farbe is singular ("it"), the verb takes the matching third-person ending: gefällt.

Why does the sentence use euch instead of ihr for "you all"?

The verb gefallen strictly requires the Dative case for the person experiencing the liking. Euch is the Dative form of ihr (you all). It helps to imagine the sentence literally asking: Is the color pleasing to you all?

I learned that some verbs change their vowel. Is that why it is gefällt and not gefallt?

Exactly! Gefallen is an irregular verb. When used with a third-person singular subject (like er, sie, es, or die Farbe), the a changes to an ä.

Could I also say Gefällt die Farbe euch?

Yes, that is grammatically correct. However, German strongly prefers to put pronouns (like euch) right after the verb, before full noun phrases (like die Farbe). So Gefällt euch die Farbe? sounds much more natural.