The game has four endings: Grey, Brown and two Pink endings.

The quizzes don’t matter, so you can disable them in the settings to save time.

The choices before the scene where Ruka gets bullied for the second time don’t matter.

Save 1 at the choice between ‘‘Because, I owe you.‘‘ and ‘‘Because, I’m your ‘‘knight’‘…‘‘

Choose ‘‘Because, I’m your ‘‘knight’‘…‘‘

Save 2 at the choice between ‘‘Just stay like this…‘‘, "Just like last time..." and ‘‘More than last time…‘‘

Choose ‘‘Just stay like this…‘‘
Choose ‘‘...Jes!‘‘

Save 3 at the choice between ‘‘...I can’t leave this apartment!‘‘ and ‘‘...Should I at least see the gate?‘‘

Grey ending

Choose ‘‘...Should I at least see the gate?‘‘

Brown ending

Load Save 3

Choose ‘‘...I can’t leave this apartment!‘‘

Pink ending #1

Load Save 2

Choose "Just like last time..."
Choose ‘‘...Jes!‘‘
Choose ‘‘...Should I at least see the gate?‘‘

Pink ending #2

Load Save 1

Choose ‘‘Because, I owe you.‘‘
Choose ‘‘More than last time…‘‘
Choose ‘‘...Jes!‘‘
Choose ‘‘Should I at least see the gate?‘‘

https://gamefaqs.gamespot.com/pc/212113-the-expression-amrilato/faqs/77451/walkthrough and my own experimentation.

Thanks to user sumwun for reminding me about the other Pink ending.