Through this panel you can also enable or disable individual pens, so you can exclude certain pens from drawing. Use right mouse button on the pen panel to show a popup menu which will help you to disable or enable pen groups.
For HP-GL/2 raster device with PC (Pen Color Instruction) support enabled there are displayed three columns of pen colors marked D, C and L. The D column shows the default colors (can be changed in the Pen Colors... dialog), the C column shows the current colors (as set by HP-GL/2 data, e.g. by PC instruction) and the L column shows the last used colors, i.e. colors lastly used for drawing.