GUIStyleState.textColor  

// Sets the text color to yellow of customStyles[0] when an
// element is turned on and pressed down
function OnGUI() {
    if(GUI.skin.customStyles.Length > 0)
        GUI.skin.customStyles[0].onActive.textColor = Color.yellow;
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void OnGUI() {
        if (GUI.skin.customStyles.Length > 0)
            GUI.skin.customStyles[0].onActive.textColor = Color.yellow;
        
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def OnGUI():
        if GUI.skin.customStyles.Length > 0:
            GUI.skin.customStyles[0].onActive.textColor = Color.yellow