The light will only see the colour ink it is attracted to as different wavelengths absorb into different things. It is not possible to produce a laser that can see all colours as some wavelengths are too shallow when absorbed to affect the ink, others can absorb into other matter and damage the skin in the process. Another reason is that all ink pigments contain certain levels of heavy metals. Some pigments may have a higher concentration of some of these than others, and these heavy metals can react with the laser light and carbonise in the skin, turning the ink black. It is very difficult to remove the ink when this has happened so, if you have other colours you would like treating, the laser operator will perform a test patch for you to see if it is possible before proceeding.