Hydrangeas

This is a classic "either–or" question, and should not have been asked. For a chemist who isn't a gardener, it's also a trick question, as acidic solutions turn litmus paper red (and alkaline solutions turn it blue).

In any case, according to Wikipedia: "In most species the flowers are white, but in some species (notably H. macrophylla), can be blue, red, pink, light purple, or dark purple [, depending on the acidity of the soil.]" In other words, most hydrangeas are white, irrespective of the acidity of the soil.