as others have said yellow is the "commando plug" color code for 110V and its common practice to carry this code through into the leads but iI've never seen it stated as a requirement to do so.
MK (under the duraplug name) used yellow for RCD protected 13A 240V leads at one stage, not sure if they still do.
It dosent really matter what colour the sheath of the cable is, as long as the conductors are corrrectly identified within, and the correct plug/socket for the intended voltage is used
It's probably advisable to have the cable a colour that stands out against its background in some instances, such as lawn mowers are probably better with bright orange cables which stand out against grass.
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