“Hair no more streaked with grey than was becoming” – meaning & grammar

I was reading Jeffrey Archer's Kane and Abel, where on page 191 I found this line:

William began to be aware of his housemaster's wife during his last two terms at St. Paul's. She was a good-looking woman, a little slack around the stomach and hips perhaps, but she carried her splendid bosim well and the luxuriant dark hair piled on top of her head was no more streaked with grey than was becoming.

Now, my problem is with the bold part of the passage. What is the meaning of this comparative sentence? We know that the hair is dark. And we don't know how it was becoming. Is it right to analyze that there was no trace of grey in her hair? Or is it slightly grey?

Please help me analyze the meaning grammatically.