The pen number (colour index) PGPLOT uses for plotting. The value should be between 0 and 255. Usually the first 16 pens are predefined to have the following colours:

• 0 – background,

• 1 – foreground (default),

• 2 – red,

• 3 – green,

• 4 – blue,

• 5 – cyan,

• 6 – magenta,

• 7 – yellow,

• 8 – red $+$ yellow (orange),

• 9 – green $+$ yellow,

• 10 – green $+$ cyan,

• 11 – blue $+$ cyan,

• 12 – blue $+$ magenta,

• 13 – red $+$ magenta,

• 14 – dark grey,

• 15 – light grey.

It is possible to change the colour representation of any of the pen colour indices using the PALETTE application. []