As Colin hurries along the secret passage with the other students, his mind is in turmoil. The school is under attack. It's his duty as a student and as a Gryffindor to help defend it. But he is too young, they say, and needs to leave with the others. Youth never stopped any great heroes, Colin thinks. Especially not Harry.
Just as Colin is about to turn back, he spots his younger brother through the throng. Dennis' face bears an unaccustomed look of terror. What will he do if Colin goes back and fights and dies? He'd be devastated. The two of them were close from the day Dennis was born; it would be like losing a limb.
Colin hovers near the edge of the crowd, conflicted. Then he thinks to himself: What would Harry do? and suddenly the decision is easy. He slips into a small gap in the wall of the passage, Disillusions himself, and heads back to the castle.
Even later, after his Blasting Curse takes Macnair out of the fight, after he spins to see another Death Eater's Killing Curse barreling towards him, as death finally claims him, Colin knows he made the right choice.