I agree with Kalleheikki and would ask the client for an RGB sample. I would caution that it’s possible that the person you have direct contact with may just take a CMYK file, convert it to RGB and send it to you. That scenario would replicate what you could do on your own BUT wouldn’t solve the problem.
I would also preface by saying that you’ll never be able to match a CMYK color because it’s intended for printing on a particular proofing substrate (paper).
All in all, I’d follow the RGB guidelines stated in the style guide. More than likely, it’s the color that the client’s designers, ad agency, and or design firm intended for anything that is going to be viewed on a screen.
Best,
Robert Olding
Studio Eight | Director of Photography
https://www.studioeightmn.com
Minneapolis, MN