"I think that the yellow one is prettier."
Translation:Creo que la amarilla es más bonita.
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.
You need to use "es" instead of "está" because it is a long-lasting attribute.
"Estar" is usually used for adjectives that can change in the moment. This includes "suci@". They also represent your feelings, like "felíz" and "triste". Remember: where you are and how you feel, always use "estar".
"Ser" is used for adjectives that indicate a long-lasting attribute, like "bonit@". They could change in around 10 years or so, or not at all. Think of these as traits that you naturally have. They define what you are.