TweenColor.Begin creates a new tween, letting you specify which color to tween to. The "from" color is the current color of the sprite.
The warning means you are not using the Widget Tool to create your widgets, or that you are adding them to a game object whos layer is different than that of your parent UIPanel. Use NGUI's functionality for everything and you will avoid this problem. Alt+Shift+N adds a new child game object.