Ajisai comes in various colours - white, pink, blue, purple, red.
I was told that the colour changes during its blooming period. Yet, I read that the colour depends on the soil composition and pH. Alkaline soil, meaning pH above 6, will produce pink flowers, whereas an acidic soil, with pH below 6, usually produces blue flowers.
Ajisai is written in kanji as well: 紫陽花, purple + sun + flower.
Because it changes the colour, it is also called nanahenge 七変化 seven + changes + flower.