Dirty, I suppose usually may be more easily associated with mud or grease, hence black/brown usually come to mind. Eg. Dirty in mud:
http://www.youtube.com/watch?v=MroZUqhgL5w&feature=rec-fresh+div-r-5-HM
But if red is the preferred choice of colour for "dirty", red which may be usually associated with blood - having a blood stained knife be bloody bandage (place at right place), or wound that look like cuts may complete the mood.
http://www.youtube.com/watch?v=MroZUqhgL5w&feature=rec-fresh+div-r-5-HM
But if red is the preferred choice of colour for "dirty", red which may be usually associated with blood - having a blood stained knife be bloody bandage (place at right place), or wound that look like cuts may complete the mood.