Mistakes in choosing paint colours will happen. Colour that seems ideal on the little paint chip or while it was in the bucket may turn out to be a nightmare once you test the colour on the wall or object. This sort of unhappy surprise often occurs because of differences in lighting in your home, compared to the lighting at the paint store and paint technician mixing mistakes. Two methods are used to try to correct this problem and lighten eggshell finished enamel paints.
Preserve the finish. "Glossiness" in paint is chemically added by the paint technician or the manufacturer according to a recipe. If the paint you purchased has an eggshell composition, just a bare hint of "gloss" one step above a pure matt finish, you cannot mix any other type of "finish" with this paint. If you mix matt, satin, semigloss or glossy paint with the eggshell paint, you will no longer have an eggshell finish at all. If this finish type is important, preserve it by only using identical finish paints in corrective measures.
Mix in only enamel type paints. "Enameling" is a chemical property as well, and you must match it if you want to maintain an "enamel" surface. Enamel paint is often oil-based but can be latex or water-based as well. Do not mix water-based paints with oil-based paints or vice versa. Nor should you mix a non-enamel paint with any paint labelled as "enamel."
Correct the colour by mixing in white paint while the "bad" paint colour is still in the can. Stir the original paint colour thoroughly, making sure to scrape the bottom of the can and bring all the colourants to the surface for proper mixing. Measure out one cup of the paint from the bucket. Add one tsp of white to this test batch. Mix the paint, and test it on a wall or a piece of scrap board. Let dry. Evaluate the new colour.
If the change isn't satisfying, add another tsp of white to the test batch, stirring and testing as before. Continue until you achieve the colour you desire. You now have a "formula" to follow: "7 tsp of white per 1 cup of colour," and can mix the entire quantity of the errant paint to match this ideal tint.
Change the colour after it has already been applied on the surface and has dried. You can over-paint the object with a second, lighter coat of thinned white paint. If the paint is oil-based enamel, add oil-based paint thinner to create a "glaze." Add water or latex paint thinner to latex-based enamels. Test the glaze over a discreet area, and let it dry. If the effect isn't enough to "take down" the original paint colour enough, add more white paint to make the glaze less transparent. If the glaze is too dark, add more thinner. Find the perfect formula, and apply the glaze over the entire object evenly. The original colour will still show through but it will be perceived to be tinted lighter.
Before you experiment with mixing colours, contact the store where you bought the paint. Many paint stores will take returns on custom colours as a customer service feature. If you feel the technician mixed the paint wrong, make the attempt to get a free remixing. You normally forfeit this opportunity once you have mixed a second paint colour with the paint.