Hypov8 has sent in a pak file which contains green weapon sprites for the grenade launcher, rocket launcher and flamer.
This means that, in game, you see green rocket trails, green flames from the flamer and green explosions when a grenade explodes.
You can download Hypov8's green weapon sprites pak file from here.
The coloured sprites are in a pak9 file. Just extract/unzip the file to your Kingpin folder. This will place the pak9 file in your kingpin/main folder.
If you already have a pak9 file in your main folder then extract the new pak9 to a temporary folder somewhere and rename it to another number such as pak2, pak3 etc which you don't already have. Then copy it to your kingpin/main folder.