Unless the designer is paying you, you should go with what you like.

What is the designers reasoning for a light cream? Designers can visualize the space when its done, that's why are designers. Find out the reason,and also ask the designer she does not suggest dark grout.

walls light ,floors dark..dark grout on walls will close in the space and light on floors will highlight dirt

The Designer's choice is her suggestion based on her opinions of good design. it's not written in stone. Ask for her justification and then decide...........but in short, unless she is paying for the project and living there, I would go with what you want, right or wrong in the eyes of the designer.