GUISettings.selectionColor  

// Sets the selection color to cyan
var str : String = "This is a string with \n two lines of text";

function OnGUI() { 
    GUI.skin.settings.selectionColor = Color.cyan;
    str = GUILayout.TextArea(str);
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public string str = "This is a string with \n two lines of text";
    void OnGUI() {
        GUI.skin.settings.selectionColor = Color.cyan;
        str = GUILayout.TextArea(str);
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public str as string = 'This is a string with \n two lines of text'

    def OnGUI():
        GUI.skin.settings.selectionColor = Color.cyan
        str = GUILayout.TextArea(str)