HP L 5 manual Shading Fill Type command FT10, the shading levels are

Note	In the “shading” Fill Type command (FT10;), the shading levels are
	mapped between white (0% shading) and the equivalent gray level for
	the currently selected pen (100% shading). In the “HP-GL/2
	user-defined” Fill Type command (FT11;), each pixel in the RF pattern
	is rounded to white or black based on the equivalent gray level of the
	pen number for that pixel. This rounding should use a low enough
	white/black threshold so that yellow will round to black.
	For a black and white device, pen 0 defaults to white; all remaining
	pen colors default to black.
	This command is ignored if the current palette was created by the
	Simple Color command (?*r#U). An IN command defaults pen colors
	as indicated in the previous table.

7-16 Color Vector Graphics (HP-GL/2) EN