PKH67 is a good one for labeling. It will be problem if you need to label two kinds (sources) of exosomes. Both EGFP/RFP protein and luciferase-labeled exosomes will solve this kind of problem. In addition, you may quantify, image/track exosome homing in vivo if exoosmes are labeled with EGFP/RFP or luciferase.