Ryan H. (eldeeem) made this exciting LEGO mosaic:
Thrilling, isn't it? But actually, it gets much more interesting when looked at under UV light:
All of those bricks are regular red bricks, not dark red or anything special. It's just that some red bricks glow more under UV light than others do. Strange, huh? Time to start making some secret messages!
There is another comparison of colors under UV light that Ryan did here.