Mediums that help alter pigments:
Ox gall - improves flow of washes, but if overused will create dull washes.
Gum Arabic - already an ingredient in the paint. adding more will increase the transparency of the paint. It can also help remove some color. If overused, it will create a gloss on the surface of the paper.
Texture medium - can be added to paint or applied directly to the paper for an increased textural appearance. Use old brushes and clean them immediately after.
Iridscent medium - adds glitter effect to painting.
Granulation medium - creates a "mottled" effect when added to wet paint.
Winsor and Newton Mediums
permanent masking medium
ox gall liquid