Dye packs would be a new upgrade for APS and CRAM shells.
For APS they're a cosmetic variation on the Visible Tracer, having the same mechanical effects but changing the color of the splash the shell makes when it hits the water.
For CRAM, they are added in the fusing box and are purely cosmetic, again, changing the color of the splash the shell makes when it hits the water. (Maybe they take a little bit of the shell, like a fuse?)
This is inspired by the real life use of shell dye packs during WWII.