Under GamePlay Constants, near the bottom, you'll find FrostAttackSpeedDecrease & FrostMoveSpeedDecrease. Setting both to zero will eliminate the slow effect you're seeing (movement, not art); however, I believe it will affect ALL frost related attacks (not necessarily spells) as well as play the cold-buff animation on the attacking unit.
You can change the buff-art used by editing the Slowed buff (under Special) via the object editor's buff tab, but the units will still be dyed blue.
You would be better off basing your spell on Inner Fire, as AYF suggested though.
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