In my installation (centos7 odoo9)

CSS file in:

/usr/lib/python2.7/site-packages/openerp/addons/web_calendar/static/src/less/web_calendar.less

Change:

rgb() clor to something darker e.g. : rgb(52, 192, 52)

@nb-calendar-colors: 24;
.o_calendar_container {    height: 100%;
    padding-top: @odoo-horizontal-padding/2;
    padding-bottom: @odoo-horizontal-padding/2;
    .calendar_color_loop(@index) when (@index < @nb-calendar-colors+1) {        .o_calendar_color_@{index} {            color: #000;
            background-color: spin(rgb(52, 192, 52), 360/(@nb-calendar-colors+1) * @index);
            border-color:  spin(rgb(52, 192, 52), 360/(@nb-calendar-colors+1) * @index);
            opacity: 0.7;
        }        .calendar_color_loop(@index + 1);
    }    .calendar_color_loop(1);