Palette swap
Reusing the same sprite or tile with a different color lookup table to get new-looking graphics almost for free.
On hardware with indexed color, each pixel stores an index into a palette (CLUT) rather than an RGB value. Swapping the palette recolors the whole image instantly — the classic way old games made a second enemy (green Goombas, red Koopas), a "night" level, or player two's outfit without spending scarce ROM on new tiles. Street Fighter's alternate-color characters are palette swaps.
The same mechanism enabled palette animation: cycling colors in the table so water shimmers or fire flickers while the pixels never move — a trick that cost zero CPU. The downside, long a running joke, is the "palette-swap boss": the same sprite recolored and presented as a tougher foe.