Because of the variation in the evenness of pigments and photo quality these are inherently approximate. My admittedly unscientific method was to sample pixels in a section of tincture until it looked about right, then drew a line in the sampled tincture over the section; if the tincture didn’t stand out against its surroundings I considered it a close enough match.
For the palettes with accent tinctures, those are the tinctures used to highlight or lowlight parts of charges and/or were used for diapering.
Pingback: To Colour or Not to Colour: a treatise on digital heraldic submissions | konstantia kaloethina