There was twisted humor to be found in the situation.
He supposed that the corpse of his fellow classmate wasn't supposed to elicit an amused (if confused) reaction, but as Hajime looked at the rapidly cooling body of Byakuya Togami, he was simply unable to take the situation seriously.
The blood wasn't supposed to be pink. It was always red. It couldn't be not red. As it was, Hajime couldn't help but to construct a humorous scenario in his head, where Togami ate too much and then accidentally overdosed on Pepto-Bismol.

The boy interrogated his classmates on the matter, addressing the elephant in the room, and yet they saw nothing wrong with the fact that Togami's blood was pink, the kind you could normally see on neon signs in nightclubs or hideous clothes in thrift shops.
Eager to prove the point, Hajime impulsively grabbed a knife and sliced his thumb open, watching in amazement as pink viscous fluid slowly oozed out of the gash. This, obviously, caused a couple of disbelieving glances (notably, Tsumiki freaked out and immediately scolded Hajime for harming himself without a hint of her usual meekness, as she tended to his wound) and comments doubting Hajime's sanity, and honestly? Now Hajime wasn't sure himself.
Perhaps he was insane. Perhaps his perception was compromised in some way. Perhaps he merely saw the blood as pink, and subconsciously substituted the word "red" with "pink" whenever it was mentioned in association with blood.

It wasn't until the trial had already ended that he decided to attack the problem from a different angle.
He asked Fuyuhiko and Akane what adjective they would use to describe the shade of pink that was particular to the liquid, and stared in disbelief when he heard "electric" or "neon" instead of the expected "crimson".
He questioned Mikan on the differences between the arterial and venous types of blood, and simply couldn't hold his jaw shut when the nurse casually admitted that venous blood was actually bright purple rather than the dark blue he expected.
He asked Soda what color were toys meant for girls were likely to be produced in, and was shocked to hear "yellow" in response.
He even questioned Mahiru about the moods that she associated with different colors, and, needless to say, he couldn't reconcile what he knew about the world with what he heard.

Hajime's world was slowly crumbling apart as he questioned the very basics of his world. What was real? What wasn't? How could he rely on his common knowledge? In this bizarro world, stop signs might as well have been acid green, and he wouldn't be able to tell the difference.
So he was very relieved when Monokuma announced that this world was "just a game", when the Future Foundation bailed them out of the simulation, hoping, just daring to hope, that when he woke up, everything would finally go back to normal, everything would conform to the unshakable foundation of human existence that was common knowledge.

And for the few days after he escaped from the simulation, defeating Enoshima AI and his own alter ego in the process, he was at peace. No longer did he have to worry about the colors of traffic lights or roses or wine or tomatoes or strawberry jam. Free of the Neo World Program, his classmates were finally starting to make sense, the kind of sense he could find himself agreeing with.
At least it was until he accidentally nicked his finger on a sharp piece of metal while helping Soda with repairing some of the machinery. He stared at the liquid lazily flowing out of the cut. The blood was pink.