Yes, a regular paint brush. Get them in different sizes and different thickness on the brush itself to get different results. Dip brush in cocoa butter, use brush in mould. Let it set before you take the next color. In this case it's probably colors without white color added, which will let them flow into each other.
You can also mount a small sponge brush on a tool like a Dremel to get a kind of whirlwind effect.