I noticed the same thing on some of my Daffs. I beleive it may be a little bit of tip burn from cold/freezing temps. Since my Daffodil bulbs started emeging, we've experienced several episodes of very low temps.

Have you had pretty cold temps lately?

It is sunlight deprivation. You don't need to cover daffodils with that much. A regular amount of mulch is fine. For bulbs that are questionably hardy, you would want a thick layer of mulch like you did for the daffodils. Anyway, you need to remove the excess leaves.
Remy

They'll green up by the next day, it happens with all the bulbs. I was out uncovering today.

Wow, a whole foot of leaves? I only had maybe an inch.

It's more than a couple days and the it is still yellow. Yes, I covered the flower bed with a foot of shreded leaves, but by mid spring, a foot of shreded leaves will be reduced to a few inches.

Don't worry. I had several bulbs (lilies, daffodils, hyacinths) which were yellow when poking through the soil. They eventually turn green.