What happens when you try to apply a blending mode to a background layer?

When a blending mode is applied to a background layer, the blending mode is disabled because background layers are typically locked by default in graphic design software like Adobe Photoshop. This means that you cannot make adjustments to how that layer interacts with the layers above it in terms of blending modes without first converting it to a regular layer.

By switching the background layer to a regular layer, you can then apply blending modes normally which alter how that layer blends with the layers beneath it. The functionality of blending modes is designed to modify the way colors interact, but this is contingent upon having the layer unlocked to allow for those adjustments. Therefore, attempting to apply a blending mode to a background layer results in it being disabled as the layer's locked settings prevent changes to its blend properties.