Invisible Ink DIY. The secret here is that the flowers act as pH indicators, changing color in an acid or a base. By using both an acid (lemon juice) and a base (baking soda), you can create a painting that reveals two different colors when you rub the flowers over it. We had a great time experimenting with our flowers. Definitely worth the sacrifice of a bouquet! Science. For Kids. Experiments. Projects. Art. Kids. Elementary. Fair. Florists. Plants. Colour. Simple. Nature.