Aseprite supports three different kind of color modes:
When you create a new sprite you have to choose one of these color modes. In this section you will see the details of each one.
RGB, or RGBA, stands for Red-Green-Blue-Alpha. Each image pixel contains these four components. It means that a pixel is completelly indepedent from others.
The alpha component specifies how much opaque the color is:
Alpha = 255 means completely opaque, and
Alpha = 0 completely
Special note: Background layer doesn't have an alpha component, so it will be always opaque.
In this mode each pixels is a number that references a palette color. Your palette can contain up to 256 colors, and each pixel points to one of those colors (from 0 to 255).
Unlike RGBA, if you modify the palette color, all pixels that are referencing that color will change their appearance.
An important concept on Indexed images is that for transparent layers, we need a special index to act as the transparent color. Generally this index is 0, but you can change it from Sprite > Properties menu.
It's more like RGBA, just that you have two channels: Value and Alpha. Here 0 means black and 255 is white. The alpha channel behaves exactly like in RGBA mode.