1. An apparatus for deterring counterfeiting of documents with a seal having a characteristic color, the apparatus comprising: a first-level detector adapted to detect an initial block having at least one pixel with the characteristic color and to thereby quickly eliminate from suspicion a majority of types of the documents without the initial block as legitimate, while identifying a minority of types of the documents with the initial block as potentially counterfeit documents; a second-level detector adapted to further test the potentially counterfeit documents identified by the first-level detector by searching for a group of contiguous blocks including the initial block, such that examination of the group for the seal can verify which of the potentially counterfeit documents are counterfeit copies; and an alarm adapted to signal detection of the counterfeit documents by the second-level detector.