Over time some jewellery pieces may change slightly in colour. This is normal for metals to tarnish due to the surrounding environment. Some things which can affect the metal can be contact with water, moisture in the air, perfume, cosmetics and other harsh chemicals. Even the natural oils on our skin can cause tarnish. When applying perfume, spray it on your skin first, then put on your jewellery.
A good way to maintain the original quality, is to grab a soft tooth brush, detergent and warm water, and give the piece a good scrub (as odd as that may sound). It helps to clean any grease/dirt off the surface which has accumulated during everyday wear. Don't use anything too abrasive as it can damage the metal, and make sure you dry the jewellery before storing or wearing. I also recommend getting a polishing cloth too. In saying all that, some people like the change in colour on their jewellery as in can add character, and some things age well. So really, it is just personal preference.
Oxidised silver jewellery over time can lose its black colour and eventually appear back to its original silver colour. If you want to keep the blackened appearance, do NOT use the cleaning process mentioned earlier. This is the same as gold plated jewellery. Avoid water contact if you are wanting to keep the plated colour.
If you find you have no luck with cleaning your jewellery, feel free to contact Jay via email - firstname.lastname@example.org - she will do her best to help.