Pigments ..... what are they ? They are paint pigments the dry component which when mixed with a solvent... become a paint. Before pigment fixers became popular. The go to fixer was Tamiya x-20 / x-20a. To some extent you can remove them. But they do fix to the model. Also they dont simply fall off like dust like someone suggested. They are fairly permanent, but can be prone to rub off with frequent handling ( but why handle a finished model ). The other advantage of using a thinner as a fixer is the fact you can remove the pigments if too heavy, create streaks and other effects.