A white nightgown and bedsheets are required only if a woman sleeps without underwear. Only those portions of the nightgown or pajamas which could reach the lower half of her body need to be white. Therefore, a sprinkling of colorful flowers across the collar is not a problem.
A woman who is wearing white underwear to sleep does not have to wear a white nightgown. There is a custom to continue to use white sheets even when wearing underwear, but this is custom and not law.
The underwear should really be all white. If for some reason a woman has run out of such underwear, the next choice would be a pair with a white crotch or inside layer of white. If that is also temporarily unavailable she can use the lightest available clean, non stained underwear. She should change to white as soon it becomes available, but does not have to delay or interrupt her seven clean days.