My main question to you would be "where are you gripping the penis"? Make sure you are not directly gripping the head itself. I grip a little below and while the head gets a little squished in my closed palm its nothing serious.
Also, see how fast you can get the discoloration in your head to go with light massage, this'll help show if you are doing any harm. Some discoloration will be normal, after all you are gripping hard and restricting bloodflow somewhat. Your 30 secs grips should not be able to do any actual harm generally though.