In case of DF the initial rash is a transient flushing erythema of face that typically occurs shortly before or within the first 24-48 hours of the onset of symptoms and is thought to be the result of capillary dilatation.
There is second rash also that occurs after fever is off it is characterized by maculopapular or morbilliform eruption sometimes individual lesions may coalesce and are then seen as generalized confluent erythema with petechiae and rounded and is thought to be due to an immune response to the virus