I am growing Romaine in pots and everyday I have a new pale leaf on the heads, not rotten though, just light colored and about 4 inches high. It is grown on my deck in NY.
Pale Romaine Lettuce
Certified GKH Gardening Expert
This is something called blanching. It happens when the plant does not get enough light. Some people like to do it on purpose to more bitter flavored lettuces and greens to sweeten their flavor. It can be corrected by giving the plant some more light.