Red can often be associated with anger, simple. Thank you. Also he has a Rageful form that gives him crimson hair(it would also get more wild and messy), crimson eyebrows, and crimson eyes with black pupils. He would also have a crimson aura, along with a taller height and body mass, his muscles would also get bigger(I was inspired by Broly on that part). Now after going to Endeavor's agency for a work study, he can now control his quirk. When using his Rageful form, he keeps his stature and height, but his hair, eyebrows, and eyes all remain crimson(his hair doesn't change to messy and wild). Endeavor also injected a substance into him allowing him for better control. Endeavor sees a lot of potential in him.