color change

[Collimation]

I designed a poster with a grey background and a green overlay with some transparancy as base. Photo and text added.Exported the file as .pdf for professional printing.
Send the file to a commercial print company and there the green color changed to purpel????
Any idea what could have gone wrong?

[handrawn]

short answer - you have not taken into account that the colours you see on the screen are RGB, and colours that are printed are in CMYK

some screen colours cannot be reproduced [accurately] in CMYK; I am assuming your green is one of these

you will need to make sure your PDF export options are correct for printing, to get the best approximation