Add all dry ingredients to a large bowl and whisk together.
Slowly add coconut oil and essential oil to the dry mixture, whisking immediately. Be sure not to add any wet ingredients all at once or the mermaid bath salt will start to fizz like an alka-seltzer. If using pure essential oil, add just one drop to each bowl; otherwise use 3 - 4 drops. I love DoTerra oils, the purity is unparalleled (I can't believe how you can actually smell the difference but you can. You will need much less quantity if you use those... I've had the same bottle of Lavender for a year and a half and have made countless, scrubs, bath bombs, bath salt, etc.
Separate the mixture into 3 - 4 bowls (or as many colors as you'd like to have in your fizzy mermaid bath salt).
Add 3 - 4 drops of soap coloring to each bowl (1 drop at a time) and mix well.
Allow to dry a little (for about 15 minutes) and then combine the colors together. Think of folding the colors together rather than mixing to avoid the colors becoming too mixed and muddy looking.
Store the Mermaid fizzy bath salt in an airtight glass jar.