Do a google search for minnow dye. I have seen some advertised somewhere just can't remember where. Maybe Jannsnetcraft or some place like that. Had different color's and not supposed to harm minnow.
I've seen the dye made by Vicious and another brand that I can't recall the name of right now but both are a bit pricey so I was looking for some input from anyone that has tried them or tried another method.
I used to work at dooleys and I do not remember them dyeing the minnows. They got them that way from there supplier. What we used to dye was the worms. We would dye them green by adding the dye to the container. Don