+$Knowing what happens to the human body when a shoulder is dislocated will help you achieve your goal. In essence, the injury is caused by a displacement of bones.
+$The shoulder is made up of three main bones that move around each other: the humerus, scapula and clavicle. When the humerus is moved forwards from its seat it causes an anterior dislocation, and when it's moved backwards it results in a posterior dislocation. When this happens there are a number of visual clues you can paint to indicate the injury.
+$First, the two shoulders of the subject will look asymmetric. The deltoid muscle will appear thinner and be moved downwards, and above it the clavicle and the acromion (part of the scapula bone) will protrude. The shoulder will appear rotated towards the interior or exterior part of the torso, generating an unnatural bend on the chest. These are the features that you need to paint.
+$I think that one of the most important things you have to do when adding colours is to enhance the lighting on the clavicle and acromion, to indicate to the viewer the unnatural position of the shoulder.
+$Once these elements are in place, you can emphasise the injury by adding more details, such as a rigid posture of the arm. You could also paint some bruises, but these must be in keeping with how the character has suffered the sprain.