The bug

In 1.21.1 and earlier, the missing model ignored biome colors, redstone power level colors, pumpkin/melon stem colors, potion and spawn egg colors, and so on. 24w33a has changed this behaviour for unknown reasons, and this change was not announced in the changelog.

This is a problem since the missing texture should be very easy to see, which may not be the case anymore if a tint is applied.

How to reproduce

I've attached a resource pack that makes oak_leaves use the missing model, which can be used from 1.13 to 1.21.1 to show that the missing model does not use coloring, and 1.21.2 onwards to show that it does. Alternatively, this can be tested using commands:

/give @s minecraft:vine[minecraft:item_model=""]

minecraft:vine can be replaced with any other item ID that uses coloration, such as spawn eggs. It can also be replaced with any item which does not use coloration for comparison purposes.

For a particularly problematic example, creeper spawn eggs appear almost entirely black, which will cause them to not look like there's an issue at all in many contexts:

/give @s minecraft:creeper_spawn_egg[minecraft:item_model=""]

Expected results

The missing model would appear normally.

Actual results

It appears darker due to being multiplied with the color the item usually is multiplied by otherwise.