Too dark labels
Fairly obvious. These may be readable but are not “in-spec.”
Slightly dark labels
Not as obvious as the too-dark labels.
Whether or not a label is “in-spec” can only be confirmed by a verifier, but they typically exhibit some visible characteristics.
Slightly light labels
In some cases, these are preferred to slightly dark ones for “in-spec” barcodes.
Too light labels
These are obvious.