* any open bottle of product has about a 1 year shelf life after opening or check the expiration date
* serums in amber bottles are placed in that type of packaging to avoid oxidation. Sunlight, heat, opening and closing the bottle all contribute to oxidation which reduces the efficacy of products. If it is changing color it's time to pitch it!
* mascara is a 30 day item. Yep, I know your mascara is likely a lot older. More products are formulated now without parabens. Well mascara should probably be an exception to that rule. Parabens keep out bacteria. No parabens= short shelf life. Everytime you remove that wand and put it back in the container you are taking germs with it.
* powder based makeup, especially mineral makeup is usually safe to use for up to 24 months
* liquid makeup should be discarded in one year or less
My biggest tip: wash your hands before handling your products and makeup! You can grow "critters" very easily by not using sanitary practices. Avoid putting fingers in jars, use a Q-tip or other implement to remove products.