GUISettings.selectionColor

Цвет выделения в текстовых полях.

using UnityEngine;

public class Example : MonoBehaviour
{
    string str = "This is a string with \n two lines of text";

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