blue ink

Evidence

"muḥaqqaq, because of its large size, was vocalized (ḍabt) with a different pen, and often in a different colour, such as blue."

Remarks

In deluxe Muhaqqaq Qur'ans, such as the 14th-century Central Asian example (RBD AC188), vocalization was often executed with a different pen and in contrasting colors like blue. Additionally, superscript alifs of prolongation were sometimes executed in red ink (p. 161).