Malva color change

Yesterday I picked Malva to make tea. Tastes nice and the color is yellowish at first. Then I added some lemon to make it more refreshing and surprise: it immediately turned pinkish, more resembling the malva flowers. I repeated it on a white kitchen towel, and see the difference below. I tried to grasp the kitchen chemistry behind it by strolling through Wikipedia while drinking the tea, but remained unsuccessful. Does anybody know what happens here and why the color changes? Is it any drop of pH (I can try that tomorrow) or is it something else in the lemon? 

Malva tea made of fresh flowers: before adding lemon

Malva tea made of fresh flowers: before adding lemon

The same malva tea after adding lemon. 

The same malva tea after adding lemon.