GUISettings.selectionColor

public Color selectionColor ;

Description

Цвет выделенного прямоугольника в текстовом поле.

using UnityEngine;
using System.Collections;

public class ExampleClass : 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);
    }
}