Light Green & Light Blue at the major ones that will not respond very well to the Nd:YAG wavelengths, if at all.
Example of this shown in the photo, using the Compact Q-Switched Nd:YAG Laser after 9 treatments we have successfully removed the Black, Red, & Orange ink, only the stubborn Light Blue/Green remains.
However if the end result is a cover-up then these lighter colours are easier to cover or incorporate into a new design.
Some pigments will not respond to any available wavelength and some pigments, especially modern, pastel, white or ultra-vivid pigments may respond by turning brown or black. These residues then may, or may not respond to further treatment.
Semi-permanent or micro-pigmentation pigments may be particularly difficult as they are often blends of ‘earth’ toned pigments, often containing iron-oxide compounds which commonly turn black on exposure to laser light.
It is essential that careful test patching is carried out on any micro-pigmentation inks to ensure that the pigment will respond without making matters worse.
Test patching potentially difficult colours is the only way to check response and it may take multiple visits to find out whether a particular pigment will respond.