Setting or changing the Print Darkness can be done using the Tone or Contrast command in CPCL. The Contrast command is an older command that is still available in newer printers to make them backward compatible. The Tone command is recommend if it is available in your printer model.
The format for the commands are as follows:
{command}{level}
where:
{command}: TONE
{level}: select a value from -99 to 200
Example:
TONE 100
{command}: CONTRAST
{level}: contrast level.
0 = Default
1 = Medium
2 = Dark
3 = Very Dark
Example:
CONTRAST 1
See the CPCL programming guide for additional information