The Indus Dictionary Project
My hypothesis is that cross-hatching represents the colour of red ochre¹.
Mohenjo-daro: Ivory Rod: M-2090 A and B: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 121: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
The example of this motif appears on a love token in the form of a hairpin. 4,000 years later, the colour red is still associated with marriage ceremonies in Pakistan. An example of this link is the henna ceremony, which leaves the skin stained a reddish brown.
Ochre: the Colour: Lynn Fawcett, 2017.
1. To read more about the link between cross-hatching and the colour of red ochre, go to the Cave Script Translation Project website: https://www.cavescript.org/research/quantum-mechanics-meets-pictographs/ochre-the-substance/.