"A color flush" is a mahjongg term which means that your entire hand of 17 tiles all belong to the same suit, for example they're all tiles with the "ten-thousand" character.
「清一色」原來是麻將術語，指十七張牌全都是同一類。例如全是萬字牌，所以被引申為全部一樣的形容詞。