Which among the following is the first change in a dead body during decomposition?

The first external sign of putrefaction in a body lying in air is usually a greenish discolouration of the skin over the region of the caecum (right iliac fossa), which lies fairly superficially, and where the contents of the bowel are more fluid and full of bacteria. The colour results from the conversion of hemoglobin of blood into sulphmethemoglobin by the hydrogen sulphide formed in the large intestine and escaping into the surrounding tissues. The colour appears in 12-18 hours in summer and in one to two days in winter. Ref: The Essentials of Forensic Medicine and Toxicology by KS Narayan Reddy, 27th edition, Page 145.