HP L 5 manual Mode 0 Indexed by Plane

You need one plane or one bit/pixel for each power of two colors in the palette. For example, a 256-color palette requires 8 planes or 8 bits/pixel (28 = 256).

Note	Direct pixel encoding modes (2, 3) cannot be compressed using
	raster compression modes 1 or 2. Planar pixel encoding modes (0, 2)
	cannot be compressed using raster compression mode 5.
	MODE 0: INDEXED BY PLANE
	In mode 0 (default), successive planes of data are sent for each raster
	row. A plane contains one bit for each pixel in a row. A pixel is not fully
	defined until it has received all the planes for that row. The planes in a
	row form index numbers that define a pixel by selecting a palette
	entry. Assuming 3 bits per index, the underlined column of bits below
	is the palette index for pixel 3 of row 1 (i1 is lsb; i3 is msb). Note that
	the Transfer Raster Data by Plane command (?*b#V) is used for all
	planes except the last plane of each row, which uses the Transfer
	Raster Data by Row command (?*b#W).
	?*b#V	row 1	plane 1	i1	i1	i1	i1	i1
	?*b#V		plane 2	i2	i2	i2	i2	i2
	?*b#W		plane 3	i3	i3	i3	i3	i3
	?*b#V	row 2	plane 1	i1	i1	i1	i1	i1

EN Using Color Modes (Color LaserJet, 5, 5M, DeskJet) B-9