Hi can anyone offer some advice on secondary colours on icing sheets please?

I am trying to print images with secondary colours such as purple, magenta etc but the colours never print true. I am aware that icing sheets are different to regular paper etc and from what I've researched all over the internet I'm not the only person with this problem and I can't find any advice anywhere on this subject. All of my other colours print out great but secondary colours are so hard to get correct. I'm wasting so many icing sheets trying to print testers! And I've tried testing on normal paper - on normal paper colours print out great but normal paper is different to icing sheets, does anyone know how to tweak the colours to get purple? Omg purple is the worst colour to get correct! Any help would be greatly appreciated! Thanks :)