Your colour choices when painting interior window sills are virtually unlimited. You can opt to blend them with the walls, contrast them or even highlight them with primary hues in a bohemian style. You are more constrained in the type of paint you use, however. Regular wall paint seldom works well on window sills. It doesn't have enough body to protect the wood and, because sills present a horizontal surface, it's a magnet for dirt. You need a gloss or semi-gloss acrylic or latex enamel that levels out to form a smooth surface and is easy to clean.
Bring a sample of the wall colour with you when you go the paint store to buy paint for your window sills. If you have the wall paint, dab a little on a piece of white cardboard. You can also bring the colour swatch, if you have it. The sample will make it easier to find an exact match or an appropriately contrasting colour.
Make your selection from available brands of latex or oil-based, or alkyd, enamel. Choose a semi-gloss or satin product for a muted finish and a high-gloss product if you want the sills to stand out. Flat enamel tends to collect dirt and isn't suitable for window sills.
Compare your colour sample to the swatches for that type of paint. Hues differ from manufacturer to manufacturer, so examine the swatches from several brands. If you know the brand of wall paint in your house, you'll probably find the best match by looking over the swatches from that particular brand. If you have the swatch and you want an exact match, show it to the store clerk and ask for a can of matching latex or alkyd enamel.
Add a can of latex wood primer to your purchase if you are painting the sills for the first time. You can use latex primer even if you decide to use oil-based enamel for the top-coat.
If you have flat wall paint and paint your window sills to match, the colours may not be identical because of the difference in sheen between the two types of paint. If you want to stain your sills and coat them with a clear finish, use varnish or polyurethane. Water-based polyurethane won't yellow as much as varnish or oil-based polyurethane.