Paint would be simplest but especially around the neck you'd want to see if you could stretch it out before/during painting. The neck area at least is likely going to be elastic and if you don't paint it while stretched the paint will constrict it after drying.
Your next option would be bias tape which may look a little nicer and move with the fabric more.
The last option is to literally make a t-shirt from scratch and use the yellow elastic from the get-go, but for basic items like t-shirts I think making it yourself is just giving yourself unnecessary extra work, unless you're like competing or something.
Depending on how much work you're putting into everything else I'd just go with paint or bias tape.