Copy the command to the command block and activate it

And rename resulting firework to: 12cal ; 762mm ; PG-7VL

Take the firework in the second hand and reload the crossbow


12cal
/give @p firework_rocket{Fireworks:{Explosions:[{Type:0,Colors:[I;15790320],FadeColors:[I;11250603]},{Type:0,Colors:[I;15790320],FadeColors:[I;11250603]},{Type:0,Colors:[I;15790320],FadeColors:[I;11250603]},{Type:0,Colors:[I;15790320],FadeColors:[I;11250603]}],Flight:1}}

762mm
/give @p firework_rocket{Fireworks:{Explosions:[{Type:4,Colors:[I;15790320],FadeColors:[I;11250603]},{Type:4,Colors:[I;15790320],FadeColors:[I;11250603]},{Type:4,Colors:[I;15790320],FadeColors:[I;11250603]},{Type:4,Colors:[I;15790320],FadeColors:[I;11250603]},{Type:4,Colors:[I;15790320],FadeColors:[I;11250603]}],Flight:6}}

PG-7VL
/give @p firework_rocket{Fireworks:{Explosions:[{Type:1,Trail:1,Colors:[I;11250603,4408131,14602026,15435844,15790320],FadeColors:[I;4408131,14602026,15435844]},{Type:1,Colors:[I;11250603,4408131,14602026,15790320],FadeColors:[I;4408131,15435844]},{Type:1,Colors:[I;11250603,4408131,14602026,15790320],FadeColors:[I;4408131,15435844]},{Type:1,Colors:[I;11250603,4408131,14602026,15790320],FadeColors:[I;4408131,15435844]},{Type:1,Colors:[I;11250603,4408131,14602026,15790320],FadeColors:[I;4408131,15435844]},{Type:1,Colors:[I;11250603,4408131,14602026,15790320],FadeColors:[I;4408131,15435844]},{Type:1,Colors:[I;11250603,4408131,14602026,15790320],FadeColors:[I;4408131,15435844]}],Flight:3}}