When use mast strobe light?
You are incorrect. Read your colregs.
Red flares (both parachute and heand held) are distress.
Orange daylight smoke is distress.
white is a signalling/alert device used by yachts, shipping (including pilot boats).
Green flares are used by search and rescue craft (especially aircraft) to acknowledge that they have seen your distress signal and have relayed information to crews that are better placed to get to you. In daylight, an aircraft will usually waggle its wings instead.
White flares are also occassionaly used as a locater device by search and rescue services during inland searches. A white flare will be fired to signal that a rescuer is watching and the people that are in distress respond by firing one of their red flares, knowing that someone is actually looking out for it.
White flares are GOOD!