To change Gridline color in Excel, use Excel Options, apply a custom-colored border to the cells, or apply a custom line color. Full details here.

If you'd like to change your legend's border color to make it more noticeable, click the Format Legend's "Border" button and then click "Color" to display a list of colors.

You can change the color of the default gridlines in Excel from the File tab, by selecting Options, Advanced. In the Display options for this worksheet section, click the Gridline color drop-down box ...