To me, your plant is perfectly normal with no identifiable issues. Not sure what you are seeing, but the seedling is fine. The lighter colouration towards the centre is also completely normal, the chlorophyll needs time to develop and in the centre, which is the newest/freshest/youngest part of the leaf, the chlorophyll is still waiting to be activated, hence the paler colour, or if you like, they haven't developed a "sun tan" yet. If you have a look at my diary "Sweet, Sweet Seeds #2, you will see some pictures of many different seedlings, where the lighter green colouration is obvious in many of them and this may give you "peace of mind" regarding your seedling.
Hope this helps,...
Organoman.