Add the shredded soap base and coconut oil to a double boiler over medium heat, stirring frequently until melted.If you don't have a double boiler, you can fit a glass mixing bowl into a pan of water to make your own makeshift double boiler pan.
Remove the pan from heat and pour the mixture into a large mixing bowl, then add the lemon essential oil and mix well.
If you'd like, go ahead and whisk in the yellow soap colorant until you reach your ideal color. Remember that adding the sugar will lighten the color too.
Stir in the lemon zest.
Pour the granulated sugar into the mixture and stir quickly as the liquid will begin to harden as soon as you add the sugar.
Transfer the sugar scrub cube mixture to the silicone mold, pressing down firmly and filling each cavity all the way to the top. Freeze the sugar scrub cubes in the silicone mold for 1 hour to allow the cubes to completely harden.
Once they're hardened, remove the lemon sugar cubes from the silicone molds and store in an airtight glass container.