Plants thrive on water in its natural state, but they tend to react unfavorably towards salty water, soapy water, sugar water, milk and soda. Plain water, when laced with vegetable dye, provokes the most noticeable and colorful reaction.
Water naturally contains small amounts of salt in the form of ions, which keeps plants healthy. Excessive amounts of salt can interfere with osmosis, the way plants absorb water and nutrients from the soil. The higher the salt content, the more the roots have to struggle to pull water into the plant. The salt accumulates in the leaves, and in extreme cases, the leaves curl up and die.
Dishwashing liquid is often used as a homemade insecticide. The problem is that along with killing the insects by dehydration, the soap covers up the minute pores and waxy surfaces on the leaves, interfering with plant respiration.
Sugar water similarly clogs up a plant's osmotic system. At first the sugar creates a growth spurt, but eventually the mixture attracts bacterial and fungal growth. Other liquids that contain sugar, such as milk and soda, create the same problems.
Water infused with vegetable dye doesn't hurt a plant, but it does make the flowers and sometimes the leaves change color. When cut flowers are placed in the dyed water, it travels up through the stem to the petals, tinting the structures. Splitting the stem in half and placing each part in a different colored dye creates a two-toned bloom.