You can either dye the Washi pulp stock before you make paper which is called pre-dye or you can dye the paper after it has been made that is called after-dye. After-dye is applied with brushing the pigment onto the paper. You can also do a blurring method by spraying the color on or by folding the paper into triangles and dip the corners or folds into different color dye, this type of paper is called Orizome.
Sakura (cherry blossoms) pinks
The sakura (cherry blossoms) are famous in Japan in the springtime. The flowers are a deep pink but paper are dyed not by the petals but from the bark of the tree. Branches are cut before spring time and then the bark will be peeled and boiled. This is used with the pre-dyed method.
Kaki-shibu (Persimmon tannin) rust
Kakishibu is mostly used to waterproof paper and it is used on the famous paper Japanese umbrella (Wagasa).Kaki-shibu is the fermented tannin juice from an unripe persimmon and is used to waterproof, insect proof, strengthen and dye paper. Let me warn you this liquid reeks and it takes a while after it dries before the smell dissipates! Kakishibu is applied as a after-dye with brushing (it is a rust color). You have to paint multiple coats to waterproof the paper it will almost turn leathery. Kakishibu is non-toxic, keep bugs away and prevent corrosion. It is also use for dying stencils.
Kihada grows wild in the North Japanese mountains. A deciduous tree from the mandarin family that grows up to 15 meters. In Japanese, Ki means yellow and hada means skin. If you peel the buff outer bark, a bright yellow inner bark is revealed. After drying and boiling this bark it will provide a bright lemon yellow dye. For pre and after dyeing.
Akane(Rubia Cordifolia) red
The oldest red dye used in Japan that has been used since the Nara period (645-1185). The dye is made from the roots of the plant. After it has been harvested it is left to rest for up to three years. It will be soaked and boiled to extract a bright red dye. For pre and after dyeing.
This is just some of the dyes that are used with Kozo paper. More vegetable dyes are Yamamomo (brown), Shikon (purple-blue), Tsurubami (grey), Yaguruma (lilac), Kuchinashi (yellow), Gobaishi (purple-grey). The interesting part of dyeing paper is the overlay dye technique where you can create more colors and shades by dying you paper with a color let it dry and then you dye it again with another color.