Temporary and semi-permanent dyes last for different lengths of time, but the way it colors hair is the same. The dye is either made of acidic dyes that are put on the outside of the hair shaft and make their way in, or it is made out of very small pigment molecules that go inside the hair shaft, sometimes using a small amount of peroxide (bleach). The molecules are then dislodged with shampoo over time. Sometimes, these molecules will form a larger molecular complex inside the shaft, but that is also eventually removed with shampoo. These dyes don't contain ammonia, so the hair shaft isn't opened up during processing. This means natural color isn't removed, and it is retained once the dye is removed. A lot of temporary dyes are made out of naturally pigmented items, such as henna or black walnut shells.6 Most semi-permanent dyes include nitrophenylenediamines, nitroaminophenols, or aminoanthraquinones. Nitrophenylenediamines and nitroaminophenols are responsible for yellow to purple colors, and aminoanthraquinones are responsible for purple to blue colors.7 Aminoanthraquinone is a red or orange-brown solid that is needle shaped that is insoluble in water but soluble in acetone, alcohol, chloroform, and benzene.8 Nitrophenylenediamine is a dark red or red needle-shaped solid.9 Nitroaminophenol is a solid that is either orange prisms or a yellow powder.10 These three substances are mixed with each other to create the right color for the hair dye.11