Maybe you applied a darker theme? A picture would help, but in the meantime, it could be a theme, or a Battery Saving feature (but that would change the color on everything displayed on screen, not just the icons) or an Accessibility setting (vision, high contrast maybe?). Another 'feature' that changes icons is going from the normal TouchWiz launcher to the Easy Mode one, but that would change your icons to bigger, brighter, and more colorful, if I'm not mistaken.