I feel Vel'koz would be a great candidate for a chroma pack. He is so simple.
http://i.imgur.com/LX0ZC06.jpg
Something like this, this was rough little doodle I did but the idea is there
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We need several Vel'Koz chromas so next time One For All comes out we can all farm till six and then go mid and turn the lane into a rave party.
It would be kinda weird having him shoot purple projectiles while being orange.
Red'Koz? No one?... Okay, i'll go away 
Vel is one of the champions with colors in his model that heavily complement his particles. You can't make him green and still think purple particles look good. This is why I suggested Particle chroma packs a few days ago
Rinnegan Vel'koz