What does "dye content 80%" mean on Sigma Aldrich's product page for "Allura Red AC" (aka Red 40)
Many dyes are sold as powders with 80-90 % purity. It is because some dyes are difficult to purify (e.g. they are difficult to crystallize). The remaining 10-20 % are impurities, such as isomers or closely chemically related side-products, but mostly they are "volatile impurities" (i.e. water). For most purposes these impurities do not interfere, so nobody bothers to remove them. Here is MSDS of Allura Red:
http://www.vidhifoodcolour.com/uploads/gallery/14067976084allura_red.pdf