#赤 is listed under its own radical but its original form was the combination of 大 (big) and 火 (fire) written vertically. A big fire is usually “red,” isn’t it?
赤 means red and it can be pronounced as セキ、シャク、あか.
赤外線（セキガイセン）: infrared radiation
赤貧（セキヒン）: extreme poverty
赤面（セキメン）: blushing; a blush
赤（あか）ちゃん: a baby
赤字（あかじ）: a deficit, a loss
赤恥（あかはじ）: deep embarrassment
赤（あか）の他人（タニン）: a perfect stranger