It's hard to see him because he has night-ops armor(it makes them invisible to the naked eye)
This is his modified DC-17m rifle/sniper.
If you can see it he has the scar on his face. He got it while he was using this armor. Its blood splattered because Grevious wounded one of Rouge's squad-mates and tore him apart with a saw blade. Rouge was right behind the poor clone and blood splattered every where. Rouge then charged grevious with vibroblade, badly wounding Grevious. Some of Grevious's blood got on rouge, explaining the blue part.Its hard to see, but ill try to update it.
EDIT: Some of Grevious's mechanical fluid got on Rouge, explaining the blue part.