The opposite of losing brightness can be described with many antonyms, including "gaining radiance," "illuminating," "glowing brighter," or "glittering." These words denote an increase in light, luminosity, or brilliance. A surface that is polished to a high sheen can reflect light and make it appear even brighter, and an object that is bathed in light can shine with a vibrant luminescence. Similarly, words like "vibrant," "vivid," and "dazzling" can convey the notion of brightness and clarity. Whether it's the joyful sight of a rainbow, the sparkle of diamonds, or the shining sun in a clear blue sky, there are many ways to describe the opposite of losing brightness.