prison-white adjective British,
informal (of training shoes) spotlessly white, as if having been worn only indoors, esp in prison [from the perhaps misguided assumption that it is impossible to soil one's footwear in prison]
Michael Douglas's missus is usually all Hollywood glitz and glamour. But the screen siren has obviously been influenced by her time with fellow Welsh lass and chav idol Charlotte. Catherine ditched her super-expensive frocks and heels for a puffa jacket and prison-white trainers (The Sun)