CAUSES: A difference in appearance between the original paint and a touch-up become noticeable when a touch-up in applied on a newly painted surface that has undergone a change in temperature. This problem is more prevalent with latex coatings and occurs when the main body of the substrate is coated at one (usually higher) temperature and then touched-up at another (usually lower) temperature. The touch-up spot will not blend in properly.
Coatings cure differently at different temperature, with cure occurring more quickly at a higher temperature and more slowly at a lower temperature. This difference in the rate of cure also can affect the appearance of a coating film because of the time it takes the film to transform from liquid to solid, as in the case of a latex paint.
SOLUTIONS: It is impossible to know at what temperature as initial coat was applied, but most coating will accept the median temperature range suggested by the coating manufacturers’ product data sheet. Therefore, one solution is to raise the temperature of the substrate to be coated as well as the coating to be applied to a median temperature.