Tube of Color Oxidizing During Storing No Matter What

I have bought tubes of permanent hair color from Sallys. Every time I go back to touch up my roots, the color of the product has changed, oxidized, and it's brown. It's usually, originally a whiteish or clearish color when first opened, but how is it getting ruined if I've put the top back on properly and have it in a cool, dark, dry place? I understand air gets to it, but it's ruining it the whole tube almost, and I'm scared to use it. Is everyone opening a tube, partially using it, and throwing out unused product? Can it still be used if it's oxidized?