Weigh out the total amount of whipped soap you will be using. This is going to be kind of hard and clumpy, which is normal. Mix it in a large bowl with a hand mixer on a low setting to get it more pliable.
Add your grapeseed oil and fragrance oil to the whipped soap base and mix well. You will want to keep mixing the soap base until it is a light fluffy consistency.
Using a scale, separate your scented whipped soap into 4 equal parts.
Add your pink mica powder to one part and mix well. I like to start with a small amount of mica powder and then add more as needed to get your desired color.
Continue coloring the remaining whipped soap bases with yellow, purple and blue mica powder.
Lay a piece of saran wrap on the counter. Take note of how long your piping bag is. We are going to create ‘lines’ of soap, you just want to make sure that you will not be making to lines longer than the pipping bag is. Aim to make them at least 1 inch shorter.
Starting with the blue whipped soap, create a line on the saran wrap. The goal is to keep the width of the line consistent.
Now directly next to the purple whipped soap, start on your yellow line.
Continue until you have made 1 line with all the colors.
Grab two corners of the saran wrap that are parallel with your lines and fold over, to make it into a log shape.
Twist the ends on both side together.
On one end, cut off the twisted saran wrap.
Place the whipped soap into your pipping bag, with the cut end toward the pipping tip. When you squeeze the pipping bag, the soap will come out of the saran wrap and go into the tip of the bag.
Pipe the soap into your jar. I find it is easiest to twist the end of the piping bag so you have a good grip on it.