My anus is usually pink in colour. it is dark purple right now. could it just be bruised?

Hemorrhoids: Yes, you could be bruised but you would hopefully remember trauma prior to bruising. Hemorrhoids are often described as purplish in color. Check out http://goo.gl/KoW8xo for more info. However, 21yo seems kinda young for hemorrhoids so it's best that you see your FamilyDoc for evaluation. There's no need to be embarrassed as we've seen everything. Besides, we're here to help, not judge. Good luck!