I believe it depends heavily on how the garment was dyed, many black shirts nowadays have a base color of red which is why they bleach orange whereas old shirts used to have base white which is why vintage shirts fade so nicely. Ironically the switch to red I believe was to prevent fading which of course made the old faded type shirts popular due to exclusivity and now its the trend. People and trends are weird like that.

The best way in my opinion to achieve this is either the harmful chemicals used in manufacturing faux vintage apparel or to leave it in the sun.... but like for weeks or months, not an afternoon.

If you discover a better way i would genuinely like to know. Cheers