set gridmajcolor <color>
The setting gridmajcolor changes the color that is used to draw the gridlines (see the set grid command) which are associated with the major ticks of axes (i.e. major gridlines). For example:
set gridmajcolor purple
would cause the major gridlines to be drawn in purple. Any of the recognised color names listed in Section 19.4 can be used, or a numbered color from the present palette, or an object of type color.
See also the set gridmincolor command.