Lesley sharp had to wear brown contact lenses for her role.
Ruth Negga is actually considered to be Irish-Ethiopian/Ethiopian-Irish, making her Welsh accent a good attempt, as she isn't from Wales. The real Shirley Bassey was actually born in Wales, to an English mother, and a Nigerian father, which means she could choose to refer to herself as Welsh-Nigerian or Nigerian-Welsh, although people widely think of her as just being Welsh, regardless of her "mixed" heritage.