> Quote -  Actually it's much much simpler: As long as the textures of the cars, tattoos, etc, are on a white background, all you gotta do is plug in the image map into the Color Tint node.

I agree that is simpler, but the result is different. In the above image, the original tattoo is underneath, top left is Multiplying and top right is Blending with a mask. (I generated the mask within the shader, and the edges are not quite right).

Multiplying darkens the original diffuse colour by the colour depth of the tattoo, whereas Blending retains the brightness of each image map. With the bright colours in this tattoo, both are okay; but they are quite different.

"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)