I think the color is used to signal (to kids for example) that this particular egg has been already boiled. There is this functionality too.

Pret-a-manger sells cheap hardboiled eggs with raw spinach in London, my fav in-between concert food so I understand the hardboiled egg desire. OP, the Coop, Denner, Aldi and Migros (Lidl probably too) sells good hardboiled eggs, if you are stranded for them. Or, replace with a little rectangle of Coop Pronto gruyere? Great protein, too.