Extra Half-Brite (EHB) is a display mode introduced with the Original Chip Set (OCS) that allows 64 simultaneous colors on screen instead of the standard 32. It utilizes six bitplanes where the sixth bit, when set, displays the corresponding color from the first 32 registers at half brightness, generating 32 additional dimmed colors. This mode provides a memory-efficient alternative to HAM mode, particularly useful for creating smooth gradients and shaded graphics without the color constraints of standard 32-color modes.