Change GUI element texture in C# when a boolean changes.

Hello!

I'm currently working on a game with Kinect and I want to implement some player feedback.

What I want to do is change an ui element when the player lifts an arm.

I have 2 textures, one arrow without color and one with color. When the boolean "leftArmUp" is beeing set in a script, the arrow on my screen should change to the one with color. So basicily something like:

 if(leftArmUp){
 Load texture with color;
 }
 else{
 load texture without color;)
 }

Where do I start with this? I'm completly new to UI elements in Unity!