Alright it's day two of Flavour week and we're moving onto the coolest of all elements: Cold damage!
Classic cold damage usually looks like a either blue beam/wave/bolt of energy and then ice everywhere or a spray of jagged ice crystals but there's so much more we can do. If you want to mix things up a bit, consider a chill horrible fog that seeps over the target and layers it in ice and rhyme, coating anything it touches like freezing rain. For something even more gruesome, consider your spell as visible only in terms of the effect. You can't really see cold itself, so as your attack collides or your spell washes over them we see skin turn blue and black from frostbite, stone split from sudden ice formations inside, plants wither and die in seconds all without any showy flash or sparkle. For something more foreign, imagine the target of your attack partially being transmuted directly into ice, spots of skin becoming crystal clear and glasslike, walls turning from brick, to ice, and then cracking instantly. Or perhaps your attacks are like beautiful snowflakes until they collide and leave beautiful and painful frozen fractals across exposed skin and glass. Don't forget to toss in details on how it feels, is it a deep ache in the bones? Is it a horrible stinging frost? Or perhaps a numb painless cold death? The choice is yours.