How to understand “What little color there was in Black's face left it.”

Question

"Up to the castle?" said Snape silkily. "I don't think we need to go that far. All I have to do is call the dementors once we get out of the Willow. They'll be very pleased to see you, Black... pleased enough to give you a little kiss, I daresay... I --"

What little color there was in Black's face left it.

"You -you've got to hear me out," he croaked. "The rat -- look at the rat --"

I failed to parse the sentence. Usually, 'what' leads an exclamatory sentence, but the sentence doesn't end with '!'. Another thing I'm not sure is what the word 'it' refers to. What kind of sentence is it? How should we understand the sentence?

Answer 1

Remember that Sirius Black is described as having very pale skin. Here "What" is forming a noun phrase

"What little colour was left in Black's face" is a noun phrase. meaning "The little colour that was left in Black's face". Compare with "What to eat" = the things that you can eat.

The word "it" refers to "Black's face". As a whole it means Black changed from having very pale skin to being completely white. When people are scared they go pale. Black has been told something that scares him. You will need to understand the rest of the book to see why "give you a little kiss" is scary (Snape is speaking ironically.)