seedlings have uneven mottled color ?

I have some seedlings under MH light- the seeds were planted one week ago- they have their second set of leaves and are growing well- I have the light about 30 inches above canopy and holding temp between 80 degrees and 68 for low- soil is a mixture of pro-mix- with worm-castings , happy frog and 20% perlite- PH is 6.6- a few of the plants are not even in color- there are light and dark areas on leaf- not all just a couple- I have read a lot about MH light being a lot for seedlings- could this just be a fluke or a genetic trait- grown a lot outdoors and never really paid a lot of attention to plants this small, planted more then I needed and discarded the weaker ones- other then the color they look strong and are growing quickly