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   Color is a daily mystery．ROY G B.IV deals with lots of big﹣picture color questions explaining why hams are often red and why NYC taxis are yellow．Today's question：why are Jeans blue？

First，why"jeans"？Jeans twin names﹣denim and jeans﹣come from the two European ports that supplied two similar fabrics（织物）during the Middle A.ges． Gene fustian was a cotton﹣linen﹣wool blend shipped from genoa．a silk ﹣wool blend called serge de nimes shortened to de nimes or denim， was shipped out of France．B.oth fabrics gradually changed into similar cotton only blends by the 19th century． according to an article titled a short history of denim y historian Lynn Downey． B.oth were used to make men's clothing，valued especially for [their] properly of durability（耐久性） even after many washings"．

In 1873 Levi Strauss teamed up with Jacob Davis to produce jeans，they sold like hotcakes to Gold Rush miners secking a durable uniform for fortune﹣seeking Jeans rapidly came to stand for the character of the A.merican West﹣﹣rank， energetic， rebellions，and capitalistic．

While gold rush miners probably didnt care about how denim looked， they cared a lot about durability，comfort and fit，at the beginning，Straus offered his waist overalls"in two varieties brown cotton"duck"or canvas，and blue denim﹣﹣but by 1911 they'd stopped using cotton duck entirely．As historian Downey explains "once someone had worn a pair of denim pants experiencing its strength． and how the denim became more comfortable with every washing he never wanted to wear duck again； because with cotton duck，you always feel like you re wearing a tent．"

So why are jeans blue？The answer has to do with dye．Unlike most natural dyes that， when heated，penetrate cloth fibers directly indigo（靛蓝）sticks to the cloth's threads．With each washing some of these dye molecules are stripped away，taking bits of the threads with them．The process softens rough fabrics and individualizes the color．This extreme customization made every pair a second skin．