Set the alpha level for this drawable [0..255]. Note that this drawable also has a color in its paint, which has an alpha as well. These two values are automatically combined during drawing. Thus if the color's alpha is 75% (i.e. 192) and the drawable's alpha is 50% (i.e. 128), then the combined alpha that will be used during drawing will be 37.5% (i.e. 96).

[Android Documentation]