Why did some of the daisies turn white?

Some of the daisies paled (turned white) because Alice threatened to pick them.

‘Never mind!’ Alice said in a soothing tone, and stooping down to the daisies, who were just beginning again, she whispered, ‘If you don’t hold your tongues, I’ll pick you!’
There was silence in a moment, and several of the pink daisies turned white.