My color in my pictures is a bit more saturated than I'd like (there was more contrast in real life), but this is a base of Essie Under Where? with piCture pOlish Shy Violet layered over the top and then dabbed with a ball of crumpled saran wrap to get the marbled effect. (There are tutorials all over the internet for this technique; Nailed It has a nice one. That is more or less the method I used, except I was lazy and just reused one larger ball of saran wrap for all my nails. It seemed to work fine!)
Next time I try this, I'll probably dab the top layer with the saran wrap more aggressively to get a more drastic marbled effect.