I think shivans getting their own unique flak effect would be kind of cool. Something reddish that is typical to them. This way you could also distinguish between friendly and hostile flak before you fly into it.
Red is a bit over-done for Shivans. I mean, in FS1 IIRC they canonically had some more orangish shots, and the mediavps gives them some purple aspects...
I would concentrate on making it look different, shape/animation wise, more than color-wise, if anyone is going to do this.
Maybe a whole bunch of mini-blasts in the animation, going off in sequence (but fast, like a chain of firecrackers. But unlike firecrackers, not in a chain, position-wise)...
Also, one thing to note, this would require more duplicating of weapons.tbl entries, and reassigning them to the shivan ships.