Alpha

Set the alpha property of your FlxSprite to change the opacity of your sprite. Values can be from 0 to 1.

// sprite's alpha will be set to 0.5 (50% opacity)
sprite.alpha = 0.5;

Demonstration

Source

View Source on GitHub 

package;

import flixel.FlxG;
import flixel.FlxSprite;
import flixel.FlxState;

class PlayState extends FlxState
{
	var alpha:Float = 1;
	var sprite:FlxSprite;
	var alphaDir:Int = -1;
	
	override function create()
	{
		super.create();
		
		sprite = new FlxSprite("assets/bigbox.png");
		sprite.screenCenter();
		add(sprite);
	}
	
	override function update(elapsed:Float)
	{
		super.update(elapsed);
		
		alpha += 0.02 * alphaDir;
		
		if (alpha > 1)
			alphaDir = -1;
		else if (alpha < 0)
			alphaDir = 1;
			
		sprite.alpha = alpha;
	}
}