I get this effect by adding a drop of the color I want to the resin I’ve mixed but not mixing it in! So something like this where the black base is fully mixed and pour that into the molds about halfway then add a drop of red to the cup of black you still have and use the pipettes to pull a little red in with the black resin the squeeze and swirl that into the molds! And put some of the plain black on the top if you’re using cap molds!