Why is the check mark black instead of green/red?

Green Check mark

The proof control inline label, which is printed directly on the proof, contains a green check mark if the verification passed.

Red Cross

If the verification failed, a red cross is printed on the proof control inline label.

Black symbol

The label will contain a black instead of a colored (green/red) symbol if the ColorProof job was printed with a custom calibration set instead of with a GMG calibration set.