"I think that the yellow one is prettier."
Translation:Creo que la amarilla es más bonita.
Whatever "the yellow one" is, it's apparently something grammatically feminine.
I was just told that "creo que el amarillo es más bonito" is an incorrect translation of the English. (Reported.)
Yeah, it shouldn't matter what it is. Just send the report so they can fix it.
Pensar ( yo pienso) should be correct as well, even though creer (yo creo) fits better. Creer is more of an oppinion and is literally 'to believe'. pensar is literally thinking with your brain about something.
So both words fit, but in this sentence creer is preferred since its clearly an opinion.
I hope this helps.