'Snowflake' shouldn't have pink on its leaves - you've either got the variety 'Roseflake', or Hibiscus sinensis tricolor.There is another variety, though, called 'Snow Queen' which does have pink on its leaves.

Many plants use the purplish and red pigments called anthocyanins as natural antifreezes or sunscreens. In cold, sunny weather, green leaves often develop a bronze tint, especially on new growth, and variegation may turn lavender or pink.