Minor [CHEMISTRY] Cleaning foam does not work if used on a dirty tile

[APARTHEID]

Just as it says in the title. If you prime and throw a cleaning grenade at a tile that is already dirty, the cleaning foam will turn brown and be completely nullified of its cleaning powers. 

I'm not exactly sure what's causing this, but it does make cleaning gigantic messes a little more difficult as you have to clean a small spot before you get down to the work of letting the grenade do the cleaning for you.