When using /summon the firework rocket will always be black instead of the given color.

To test I crafted one by hand (purple small ball) and launched it. Then quickly used the command /data get command to get the data. Copying that as close as possible into a /summon command still resulted in a black ball. Using the /data get command on the summoned firework showed the data correctly except for the color. The crafted firework had Colors:[I: 8073150] while the summoned one just had Colors:[8073150].

For referrence here is the command I used:

/summon minecraft:firework_rocket ~ ~ ~ {FireworksItem:{id:"minecraft:firework_rocket",Count:1b,tag:{Fireworks:{Flight:1b,Explosions:[

]}}},Life:1,LifeTime:26,ShotAtAngle:0b,HasBeenShot:1b}