You actually need 6 red clues to unlock marco's route. Did you find the green clue on his bad ending a? If you did, I think you need to complete Feathor's route (to unlock Feathor I think you need all of the green clues) to find the last red clue, and then I believe you can actually play Marco's true ending. I'm not sure, tho :C I've managed to find the last green clue, but I don't know how to enter Feathor's route.
Thanks for the reply; it's exactly like you said. c: You can get into Feathor's Route from Yvonne's Bad Route (now that you have all green clues, a new option should appear there for you).
I figured it out about an hour before you posted this, orz. But I really appreciate you replying! I found a few other surprising ends along the way, so it was a good thing I tried it again on my own. There are still a couple other endings I'm missing, but I haven't spent all that much time on getting them yet. Marco's ending was really nice, I'm glad I was able to see it! Thank you!