This was introduced back by the SGPC of Basanti (xanthic) or surmai (greyish-blue as the colour during Guru Hargobind Sahib Ji.
Source: https://en.wikipedia.org/wiki/Nishan_Sahib
Arguably, some feel the original shape colour was as shown at https://en.wikipedia.org/wiki/Nishan_Sahib#/media/File:Sikh_flag.jpg
