Colours are specified by arbitrary combinations of the letters R, G and B
for red, green and blue, respectively. The same letter may appear multiple
times thus allowing for a fine tuning of colours.

Here's a "colour triangle" to show some possible choices:

 Green
 |     RGG
GGB         RG yellow
 |               RRG orange
 GB cyan                Red
 |               RRB
GBB         RB violet
 |     RBB
 Blue

Colour compositions specified this way, however, are always displayed in full brightness,
there's no way for halftone colours like brown. 
If you want to generate halftone colours, the brightness of the single components
can be controlled by specifying additional factors between 0 and 1 in parantheses after the base colours, e.g.:

   r(0.5)g(0.5)b(0.1)

gives a brownish colour ("dark yellow") with a tiny contribution of blue,

   r(0.5)g(0.5)b(0.5)

gives some sort of grey.

Although you can specify arbitrary colours the actual appearance, however,
depends on the capability of the graphics output device.
Specifically, bitmapped displays like X-screens have a limited colour table
(most often 256) which has to be shared among applications.
Thus it is not always guaranteed that you get exactly the colour you want,
but rather a colour which comes closest to the one you wanted. If this is unacceptable
you might try to close other applications in order to free allocations in the colour table.

Colour printers might have a limited capability to print fine tuned colours.
Often they resort to some dithering (overlay of differently coloured patterns),
which might look ugly especially for thin lines.
In such cases you might have to choose less sophisticated colour compositions.

Note also that monochrome PostScript devices try to represent colours
by black and white grey scales. This not always looks nice, especially for thin lines.
Do not use colours in such cases. If you don't want to change the original
dataset or the colour specifications you can use the PALETTE display parameter
to reduce the palette to monochrome.

Here are some examples for colour specifications:

   H:   X     Y,12LTRG            A thick solid line and symbol 12 in yellow
   H:   X     Y,dTr(0.5)b(0.5)12  A thick dashed line and symbol 12 dark violet