What is the 平仄 rule in 無情對
Since you just ask for tone pattern rule, the tone pattern of couplet also matches with the poetry. So I use tone pattern of middle Chinese for reference:

This is an ideal situation:


  国破山河在，
  仄仄平平仄
  
  城春草木深。
   平平仄仄平


But sometimes it's hard to match a pair of word in level (平) tones and oblique (仄) tones, so when it comes to normal couplet case, we just require the tone of last characters must match the tones. 


  上联：雄黄酒    平平仄 
  
  下联：牡丹烟    仄平平