Add water and glycerine to make a paste of the proper consistency if a paste is desired. All you need to have is some purchased melt and pour base, a double boiler, some additives such as fragrance oils, essential oils, herbs, coloring and molds. Melt over low heat, you don't want to let it get too hot, try not to let it get over 160 degrees or you will get cloudy bars. A good tip is to let half of the base melt, then remove from the heat and leave it in the water until it melts the rest of the way. Try not to stir it as it's melting, if you have to, just gently nudge the soap around or you will create air bubbles which won't make very appealing bars.
Many people use food coloring, but they can fade and bleed.
You can purchase color nuggets, micas or pigments to help achieve a better color. Stir your coloring in gently. Add any additives such as herbs when your base has cooled and started to thicken to help suspend them in the base. You can also add other ingredients at this time such as aloe vera gel, vitamin E, beeswax, cocoa butter, shea butter, etc. Melt your solids such as cocoa butter and beeswax before adding to the base.