If your sunglasses are not dark enough, you have a couple of options for making them darker. You can take them to a place that adds colour to lenses or, if you are lucky enough to have access to a lens tinting machine and know how to use it, you can retint your own sunglasses.
Verify that your lenses are the type of material that can be retinted. Glass lenses and lenses with special coatings such as anti-reflective coatings cannot be re-coloured. If you are unsure of the material and coatings, check with the place where your purchased your lenses.
Turn the tinting unit on and allow it to heat up to the manufacturer's recommended heat setting for dyeing lenses. Colour is applied to lenses the same way it is to clothes and eggs -- the colour is mixed with hot water in the tinting unit and the lenses expand and soak up the dye.
Fill the dye unit with the colour you are tinting the lenses. Tinting the existing colour darker works best; for example, if your sunglasses are grey, it is best to use more grey instead of adding brown.
Remove the lenses from your frame. If the lenses are held in place by a screw, carefully loosen the screws from both sides and take out the lenses. Lay a towel on your work surface to catch the screws in case they fall out. If the lenses are in a plastic frame, heat the frame in warm water by soaking for at least a minute to make it more pliable. If the lenses do not come out easily, do not force them. Keep reheating the frame until the lenses pop out easily.
Place the lenses in the lens holder and immerse the lenses in the heated dye. Allow the lenses to sit in the dye for a few minutes. Many things affect the length of time it takes for lenses to darken, including the age of the lens and the material and coatings on the lens. Check the lenses every few minutes until they have darkened to the shade you want.
Many opticians will darken your lenses for a minimal charge. However, most will caution you that they are not responsible for any potential damage to the lenses during the process.
Keep in mind that if you have had one of your lenses replaced and your lenses were not made at the same time, the lenses might not darken at the same rate.