Lettuce seeds are tiny and their seedlings are tiny too. So small that I couldn't get a very good picture of a newly emerged seedling. Their two cotyledons are held together when they first emerge and slowly open like butterfly wings.
Soon after sprouting these seedlings turn dark reddish because they are a red variety of romaine lettuce. It can make them a bit hard to spot when counting.
The seedlings in under the opaque white plastic and the clear plastic don't change color. They remain a lovely green with a bit of red mottling.