Wow, those were beautiful! FYI, it is only those first two "leaves" that are cotyledons - the ones you show in the first photo. Yes, it seems like there are two kinds of leaves after that, but they are both true leaves, one juvenile, one adult, or maybe they are both adult. Not sure.
The gray color you mention makes think of a spidermite infestation. Did you check for them using the tapping over white paper test? The mites are too small to see unless they are on the white paper. They are just tiny specks. The leaves would have stayed that color and just dried up, not turning brown.
