@MickRose It’s a 300mg with a watercolor finish type of paper, so not really thin, but it sounds to me a bit off that with the same input each print come out differently…if the input it was wrong all the prints they are supposed to come out wrong.
To be fair @NotMyFault I really find hard to understand the instruction of the link, this is not really my specialty so I don’t know what I’m really aiming for.
But it’s very helpful what you explained about RGB vs CMYK. I provided the PNG because I was worried about the front but now I’ve transformed the font in vectors and exported the file in CMYK with the informations they have provided. I’ll send the file and ask for confirmation…although they don’t sound very helpful!
I could also change the green colour so the K is zero and change the other components CMY. See below my current green with the K at 56 and the proposed at 0. Would this help?
Yes, that's what they sent me, some copies in black other in green.