tony, the stuff your talking about is "magic touch" you can buy it in PAcks,i ts like a film/transparacy you just feed it in the copier and it lays your image on, we stuff around with them at work all the time, there is a button on most new copiers for positive/negative image so it will print in reverse and then you just iron it on so the toner is trapped on the inside.
Cant say i have don tit on the colour units, because i still have Bw images here i have never used. but we did copy onto some t-shirt one of the guys at work so we all came in one day waring them on his birthday If i remember. BAsically they are fast and fun and about a dollar a sheet, so the longevity means nothing you just make a new one up in another colour :O)