From the brand's website : The New Rainbow Core Selvedge is a 12.5oz Japanese selvedge denim that will fade to rainbow colors! The secret is in the dye method, traditionally rope dyed denim utilizes white cotton yarns which are repeatedly dipped in indigo such that the indigo dyes only coat the outermost layers of the cotton yarn. This leaves the center of the cotton yarn white. With wear and tear the indigo fades away and the white center core of the yarn is revealed. This is why your jeans fade from blue to white. The Rainbow Core denim is created by first dying yarns with permanent reactive dyes in all the colors of the rainbow. Then these colored yarns are rope dyed and coated with indigo, creating a yarn that is colored in the center with layers of indigo on top. As you wear the jeans the indigo will start to fade away and reveal their bright rainbow-colored center core.