The Alpha value controls the opacity of a Sprite. A value of 1 (default) results in a fully-opaque Sprite, while a value of 0 is a fully-transparent Sprite. For more information see the Alpha property on IColorable.
Sprite Alpha can be also be set in Glue directly and through states.
Sprites can also be made fully invisible by setting the FlatRedBall.Sprite.Visible property to false.
The following shows how to make a Sprite fully transparent:
SpriteInstance.Alpha = 0;
The following shows how to make a Sprite fully opaque:
SpriteInstance.Alpha = 1;
The following shows how to make a Sprite half opaque:
SpriteInstance.Alpha = .5f;