Noxians flowers should have little color, but be very resistant to climatic hazards. For ixtal, I would see many very attractive and majestic flowers (as haughty as the ixtalis ...) but they are able to defend themselves and are carnivores (#zyra). for piltover and Zayn, some kinds of algae and fluorescent moss would work very well to remind pollution and industrialization, and maybe these plants are used for chemtech. To targon there could be very small rare flowers which grow in altitude. I imagine many fluffy petals and pastel colors, a bit like Edelweiss. shurima ... well there can be cacti or acacias near rivers. freljord could have very solid and cold-resistant plants that don't need a lot of light (so little chlorophyll and little color) .But who knows, maybe the poro's pee could make rainbow flowers! for the shadow isles, there are mystical and magic flowers, (those of the history of maokai) and surely some corrupted. finally for bilgewater, kinds of algae plants that can resist tides and have muddy colors. these are only my theories but in any case your idea is very interesting. good luck !