I suppose it depends on whose rules you are shooting to, but UKAHFT rules are here:
https://www.sites.google.com/site/ukahft/2017-format
I don't think there is any rule on plate size/shape/outline (seen some crazy ones) I believe the face and kill have to be plain coloured (most are yellow & black).
As it says on the website "IF YOU ARE IN ANY DOUBT ABOUT ANYTHING ALWAYS CONSULT UKAHFT"
Hope that helps