set gridmincolor <color>
The setting gridmincolor changes the color that is used to draw the gridlines (see the set grid command) which are associated with the minor ticks of axes (i.e. minor gridlines). For example:
set gridmincolor purple
would cause the minor 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 gridmajcolor command.