The effect of soil acidity on blossom colors is actually indirect. Aluminum is responsible for changing the hues, allowing anthocyanin molecules to move closer together, resulting in blue coloration. It becomes more soluble in acidic soil and, therefore, is more readily absorbed by plants. Soil acidity and the resulting aluminum uptake could also affect the pink-to-blue color change with other flowers. Some flowers, such as wild morning glories, are pink when they initially unfold and then turn blue. Why? No one knows-yet. But aluminum is toxic to plants, and death usually results at levels needed to effect that color change. Most flowers are not immune to aluminum toxicity, so keep tabs on soil acidity. To maintain a blue color, a pH of 5.0 to 5.5 is ideal.
When soils are naturally alkaline or slightly acidic, use sulfur or aluminum sulfate to turn flowers blue. Sulfur acidifies soils, increasing the uptake of aluminum that is naturally present in most soils; aluminum sulfate acidifies soils and adds aluminum. You can also increase aluminum uptake by using acidic peat and acid-forming fertilizers, such as ammonium sulfate. Avoid fertilizers high in phosphorus, which form insoluble aluminum compounds. If you want pink flowers but your soil is naturally very acidic, add limestone to increase alkalinity. The ideal pH range is 6.5 to 7.0. For a longer-lasting effect, dig limestone into the soil at planting time.
Flowers, of course, have their inherent colors. This genetically programmed color is affected by acidity within plant cells, which is unrelated to the acidity of the soil. So changing your soil’s pH won’t help in altering the appearance of most flowers.