Root. In the computer world, root refers to the top-level directory of a file system. The word is derived from a tree root, since it represents the starting point of a hierarchical tree structure.
Use a fertilizer with high levels of phosphorus. Phosphorus helps to prevent aluminum from creeping into the system of the hydrangea. Choose a fertilizer close to the ratio of 25/10/10 (Phosphorus is the middle number).
And the coloring is the reverse of that stated in the question! Lower pH generates BLUE flowers -- higher pH generates PINK flowers.   Aluminum sulfate, aka hydrangea bluing, is often used to acidify the soil and increase the aluminum availability to maintain blue hydrangea flowers.