A good thought, but I wouldn't use Floor in this case. The slightest noise on the white in your original image will come out as black. Bias is a little more forgiving.  :O)

Multiply is an alternative way to apply the tattoo. Compared to using the Blend node it will darken all the tattoo colours by the value of the underlying skin colour, and that's fine if it's the effect you want.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)