set gridmajcolour <colour>
The ‘gridmajcolour’ setting changes the colour that is used to plot the gridlines (see the set grid command) which are associated with the major ticks of axes (i.e. major gridlines). For example:
set gridmajcolour purple
would cause the major grid lines to be drawn in purple. Any of the recognised colour names listed in Section 4.6 can be used.
See also the set gridmincolour command.