My clear retainers (invisalign) make my teeth look grey and dull. What can I use?

Retainers or teeth? If your Invisalign trays or retainers have turned grey, it may be from your retainer cleaning technique. See your Orthodontist. If the teeth are discolored, see your Dentist. There may or not be an OTC solution depending on the cause, but you will have to be seen to be evaluated.
Cleaning trays. The trays are probably getting gray quickly if they are soaked in an effort to clean them. I would recommend remaking them, perhaps every six months, and simply cleaning them with a toothbrush and toothpaste. Storing them, cleaned, and in a retainer case when they are out of your mouth (not soaking in solution), will make them last the longest.
Consult with your Dr. See your dentist regarding this issue, you may use some teeth whitening the same time with your invisalign trays.
Discuss with ortho. They should not-- we have many invisalign patients and have not heard/seen this. Talk with your orthodontist.
OTC strips. Over the counter stripes is a good start. If the are strong enough there are professional strength which your dentist has available. However strips are only recommended if no anterior fillings present.
Bleach . I have found that invisalign retainer are excellent to hold bleaching gel on your teeth. I generally offer it as a bonus feature for all my invisalign patients. As your dentist if he can supply you with some gel. I find that kor is the best and results in less sensitivity.
Bleach. You can use your invisalign retainers as bleaching trays. Ask your dentist for some day white or equivalent tooth bleach and brighten your teeth.
AlignerBright. Ask your dentist for alignerbright gel. It is a carbamide peroxide gel designed to whiten teeth and keep aligners clean.