Most tattoo inks can be simply mixed together to create a new shade or adjust a current
one, even if they aren’t from the same brand. The only thing to bear in mind is that different inks will
have different thicknesses, which may affect how they combine - you can use a colour mixing solution to help with that.
For an ink that’s too thick, you can dilute it down using distilled or sterilised water. Some artists prefer
to use glycerine - a common ingredient in tattoo inks - or witch hazel, which also has soothing properties.
Ideally you’d match the carrier fluid in your ink to your dilution fluid.