Difference between contusion and post-moem lividity is that post-moem lividity-
The synopsis of forensic medicine & toxicology;Dr k.s narayan reddy; 28th edition;pg.no. 88; Contusion or bruises are not washable but postmoem lividity is washable. Also refer page no.106
Shows sequential color change
Easily washable
Shows diffuse irregular margins
Has raised enzyme levels
Mechanical injuries