Take the white soap base and melt it in the microwave. Add in the portion of fragrance or essential oil you've saved and stir well.
Then divide it into two measuring cups for the two colors.
To one (shown here), add the pre-mixed blue ultramarine and a bit of white mica. To the other, add some white, sparkly or snowflake mica. (Or whatever color/mica you like!) Mix well and you're ready to go.
Somewhat like working with a swirl in cold process soap making, you need to be very mindful of your time. However, instead of the soap reaching trace and thickening, here, it's just going to cool down and harden. If at any point the soap cools down so much that it's not stir-able, just microwave it for 10-15 seconds to heat it back up again.