rit dye is available in most grocery stores- in the laundry isle- same area of walmart/kmart/meyers& (big box stores)
if you use tea or coffee-make a strong batch- the longer it soaks the darker the (stain) will be- after rinsing you can try to neutralize the acid- with vinegar or peroxide-
then heat set. there is the possibility of the stain to fade over time with laundering- but it depends on how often it's washed- and the conditions (cold water- mild detergent..) sun also fades -- but the washing and sun has that effect pretty much on all fabrics-not just tea/coffee stained fabrics.