Last week a customer wrote that they were seeing a decrease in the color of their images in both the Flash gallery and the HTML galleries in the Web module of Lighroom. This is actually expected behavior.
This occurs partially because the original images that you’re viewing in the Library and Develop modules are displayed in a larger color space and because the images probably have more colors than the Web module’s sRGB color space can display. As the colors are moved from the larger color space, down into the smaller sRGB space (when they go into the Web module) the appear to loose saturation.
In addition, Lightroom uses the default browser for your OS to display the preview of the image in Lightroom. If the computer is using a color profile that’s different from sRGB, and the default browser isn’t color managed, the colors will be shifted in the Web module.
Additionally, remember that your web gallery will be viewed by whatever browser the visitors to your website are using. Most computers are not color calibrated and many browsers are not color managed, so most of your visitors will get varying experiences.