Here goes....... only pics after customization
It is very simple. Just get some ink from your gundam paneling marker on your fingers and start smearing away on the abs. Try to do it evenly to avoid black blotches. if smear is not evenly spread try using a little cooking oil to even out the smear. The ink is removable with alcohol so no worries if u make a mistake.
Do a few layers of smears to get the desired tone and you will have your mark III back in tip top condition. (note i did lose the scuff marks on the abs when doing this)