hey alvin! it seems that there have been problems with the UI, actually there should be instructions in the game :') but i guess my poor time management prevented me from really testing it in the end.. .
anyways: you have to use the arrowkeys (up and down for cycling through the categories, left and right for choosing a sprite in the category) for this game and then you can also change the colors with the "E" key! have fun and thank you for playing :)