The brown colour of the contusion is true is due to –
The synopsis for forensic medicine & toxicology Dr .ks.narayan reddy, 28th edition, Pg no. 108 

Colour of the bruise indicates the age of a bruise, the colour change is due to the breaking down of hemoglobin into hemosiderin, haematoidin and bilirubin by the action of enzymes. 
At first: red Few hours to 3 days: blue 4th day: bluish black to brown (hemosiderin ) 5-6th day: greenish (haematoidin ) 7-12th day: yellow (bilirubin ) 2 weeks: normal
Hemosiderin
Reduced hemoglobin
Hematoidin
Bilirubin