The bug

Items with the 2D missing texture in the inventory appear much, much darker than they previously did.

The missing texture is supposed to appear bright and obvious to highlight errors, so it appearing darker makes it much less striking.

How to reproduce

1. In an old version, give yourself an item with a missing texture. This could be air in 13w38b, or an end portal item from snapshots 14w05a to 14w21b.
2. In the latest snapshot, give yourself something with the missing model (via MC-276671):

   /give @s stick[item_model="minecraft:nonexistent_model"]

Expected results

In the latest snapshot, the inventory icon for the thing with the missing texture would appear bright, just like in old versions.

Actual results

It appears strangely dark.

How to fix

This may be due to incorrect gui_light settings being applied to the missing texture. Since the model is internal and cannot be changed in resource packs, the gui_light used should either be changed to "front" internally, or perhaps a new "none" setting could be introduced which would also be used internally for the missing model.