Use water-based colorants with melt and pour soap. There are two different methods to swirl melt and pour soap colors. The first method is to melt two separate containers of soap, color each one separately, and then carefully pour them into the mold. The second method is, if you have liquid colorant, to fill the mold with uncolored soap and then drop a different color into the soap at opposite ends of the mold. Then, no matter which method you used, gently swirl the colored soap with something like a toothpick or a craft stick. If the mold is deep, remember to swirl the soap at the bottom, too.