When shields take damage, their color change.
Seems that the problem is the color is defined by Data Value (damage).

Something similar happens when crafting shields: black shields has no damage, and white shields has 165 points of durability.

I have also found that, for example, when trying to apply a banner model to a yellow shield (DV 11), it gives a black shield. But, if it is damaged and the data value is 91, the banner cannot be applied.