Effectively, these layers of words formed a watermark, for they were just an impression against the wall, a pale hint of green or sometimes purple the only sign that once they might have been raised letters. 

1、 a pale hint of green or sometimes purple 是不是对 impression 的补充说明？

2 、the only sign 感觉好突兀，前面是不是省略什么？

3、that 引导的是什么从句？