Vielen Dank! Thank you for explaining the difference between "sie" und "Sie". In the task, it said the following

Ihre Kollegin Ayla schreibt:
„Ich habe die B1-Prüfung bestanden!“

Schreiben Sie:
Glückwünsche
Fragen zur Prüfung
Einen Vorschlag, wie ihr das feiern könnt

Since, it said "Ihre Kollegin, I was a bit confused what to address her with".

Because theperson writing the the task was addressing YOU in a polite way. For instance, a foreigner talking to you would refer to Ayla as "Ihre Kollegin Ayla". The "Ihre" is a possessive applying to you (you "possess/own" the colleague). A friend talking to you would say "Deine Kollegin Ayla".

Think about this: If someone would refer to your car by "Ihr Auto", you would not be tempted to address your own car using "Sie"....

Okay, so I should have addressed Ayla using "du", right?

Would make most sense to me. Actually, you can choose between "Du" and "du". Duden allows both if you are writing a letter. Traditionally, the form "Du" is considered the polite form, while "du" is a bit pejorative, but since the advent of text chats, people are often lazy to use the "shift" key of the keyboard, and in particular young people don't see a problem anymore when writing "du". Still, IMHO it never hurts to be polite, so my recommendation is to use "Du" when addressing someone in a letter or email etc.

With the advent of the Internet, it became common to use "Du/du" to everyone on the Net, so you would use it also here when responding to someone in German. However, since you usually don't know the age of that person, your response could be experienced as impolite if you wrote "du" instead of "Du".

Thank you for the explanation!