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