Palette swap
Also said as Recolour, Colour swap, Palette shift
The same image data shown with a different set of colours. The picture stores numbers that point into a palette, so changing the palette recolours everything at once and costs no extra art.
It turns one sprite into a family: a stronger enemy, a second player, a frozen version. On hardware with little memory it was a free character.
Say it
“Do the elite enemy as a palette swap of the grunt: swap the body red to purple and the eyes to white. No new frames, and the player learns it is the same enemy but worse.”
Where you’ve seen it
Luigi is Mario with a different palette in Super Mario Bros., and that game's clouds and bushes are the same shape in different colours. Mortal Kombat built Scorpion and Sub-Zero from one recoloured ninja.
Not to be confused with colour grading
Grading shifts the whole finished picture. A palette swap changes one thing's colours exactly, and can change any index into any colour at all.