Hibiscus mutabilis, also known as the Confederate rose
Flowers are white in the morning, turning pink during noon and red in the evening of the same day. Under laboratory conditions, colour change of petals was slower than that of flowers under outdoor conditions. Temperature may be an important factor affecting the rate of colour change as white flowers kept in the refrigerator remain white until they are taken out to warm, whereupon they slowly turn pink. (http://en.wikipedia.org/wiki/Hibiscus_mutabilis)