GUIStyle.active

Description

Rendering settings for when the control is pressed down.

// Assigns a texture to button for when the control
// is pressed down

var aTexture : Texture2D;

function OnGUI() {
    if(!aTexture) {
        Debug.LogError("Assign a texture on the editor first");
        return;
    }
    
    GUI.skin.button.active.background = aTexture;
}

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour {
    public Texture2D aTexture;
    void OnGUI() {
        if (!aTexture) {
            Debug.LogError("Assign a texture on the editor first");
            return;
        }
        GUI.skin.button.active.background = aTexture;
    }
}