The ink you stamp with depends on the colouring medium you are using. In general terms, you should use the 'opposite' type of ink for stamping when compared to the colouring medium you are using.  Here are two main methods of colouring and the recommended ink pads you should use.

WATERCOLOURING

When you are using watercolour mediums for colouring, such as water-based inks, Stampin' Write Markers, or Watercolor Pencils, you need to stamp your image in solvent-based ink, such as a StāzOn Ink Pad. That will ensure that when you watercolour, your stamped image will not bleed.

ALCOHOL-BASED MARKERS

When you are using alcohol-based markers for colouring, such as Stampin' Blends, you need to stamp your image in water-based ink, such as a Memento Ink Pad or Classic Stampin' Pads. That will ensure that when you colour, your stamped image will not bleed.