Most sublimation tee's are white, or very light coloured. To sublimate on purple you need to either use an OKI laser printer with white toner, or use the JetPro 3g Opaque transfer paper (or an equivalent).
I use the jet opaque 3g, can be quite tricky though as you need to lower the press temp and pressure for poly shirts, you can print 3g opaque at around 320f or 160c instead of the recommended 350f or 175c.
I don't have an OKI printer with the white toner so 3g is my only option, I print around 10 seconds on coloured poly shirts, lift up the press let it cool then do another press for another 10 secs.
White poly shirts using the jet opaque 3g are crackers and you can get some great looking logos etc but colors are tricky as I still get a little bleed through if to much pressure is used, I try to use as little pressure as I can get away with on darks at 160c