I always do a cold soak first, just water, before adding any chemicals, or heat. Often the cold soak shifts a lot if not all of the stain, it will never cause it to become harder to remove, whereas both heat and chemicals can cause the stain to become more permanent.
Then I'd usually try good old laundry soap next.
I agree that Terramycin spray usually seems to come out okay in a normal washing machine wash. Purple spray not so much - but does fade out over a number of washes, I think. I usually am not bothered, though, as I'm pretty careful about not going near the livestock wearing any of my few 'good' (non-farming) clothes, so such stains are generally on my working clothes, and don't therefore bother me.