plot2d + gnuplot + line colors in rgb

Hello,

I am new to maxima and would like to configure in detail the colors used by
my plot2d function.

set_plot_option([gnuplot_preamble, "set style line 1 lt 2 lc 'green' lw 2 pt
3 ps 5"])$
F(x) := max(-2.5, x-52.5)$
G(x) := 0 $
plot2d(    [F,G],
    [x, 40, 60],
    [y, -4, 8],
    [xlabel,"S0"],
    [ylabel,"Option's Payoff"],
    [gnuplot_term, ps],
    [gnuplot_out_file, "/home/julien/Documents/donn?es/finance/maxima/
plot-option.ps"]
#    [style, [lines, 3, 4],[lines, 3, 5]]
)$

I want to use colors other than those default colors provided by maxima. How
can I specify the rgb colors.

The above code does not give me other colors than the default ones.

Can anyone help please,

Thanks in advance,

Julien.