It would definitely be better to use the cream to match the tile, as the black grout will compete with your shower pattern. Black would also point out any irregular pattern to the tile, such as cuts around commodes, or vanities. The cream will allow the eye to nicely flow around the entire room, without a cut-up look,and giving it a richer look overall. Most bathrooms are not large enough to handle both the harlequin pattern and such a strong pattern that the black grout would create.
If My Shower Is Black And Cream Harlequin Pattern And My Bathroom Floor Is All Cream, What Color Grout Should I Use?
Black grout it's going to match what you have in the tub.
Personally, I would use a winter white or any off white color. Since your tile is not a bright white, you should not use a bright white. Happy decorating!