Wrong wiring color used for 240V appliance (white/black instead of red/black)
If this is a cable assembly (like NM or MC cable) NEC 200.7(C) allows re-identifying the white wire for use as an ungrounded conductor by tape, paint, or other permanent means.

If it pipe and wire then you need to re-pull the wire. Usually it doesn't have to be red, but red is good, it can't be grey, white, or green and it's easier to not pick through details and avoid anything with stripes.