Have a quick read of our anodizing instructions at
http://www.caswellplating.com/kits/aluminum.htm. There are a lot of factors during the anodizing process that can alter the color of the dye. Temperature of the bath and current applied all affect the pore size, which affects how well the dye is accepted. Too large a pore size and the dye bleeds out; too small and the dye can't penetrate.