If your fixture has HSB attributes this is done thru:
attribute "ColorHSB1" at value; attribute "ColorHSB2" at value; attribute "ColorHSB3" at value

ColorHSB1 represents Hue
ColorHSB2 represents Saturation
ColorHSB3 represents Brightness

If your fixture only has RGB(WA) attributes it is a bit harder but still possible (just not thru commandline as far as i know): just use the Autocreate option in the Setup. Choose additional presets and set the amount of HSB presets you need. Keep in mind that the console puts the newly created presets right after the last colorpreset item in the pool.

Best regards
Mark van der Meulen
Bavel, The Netherlands