The Zohar analogizes these ten sefirot: Ḥokhma, Bina, Tiferet, and Malkhut, to four colors, which are: White for the sefira of Ḥokhma; red for the sefira of Bina; green for the sefira of Tiferet; and black for the sefira of Malkhut (as stated below, Bereshit II, page 14, section 27).5The Sulam generally refers to the sefirot as the "ten sefirot," even when only referencing some of them, as he does here.
The meaning is as follows: An analogy for this concept is a window with four panes colored in these four colors, such that although the light passing through is one color, when the light passes through the panes it becomes colored and turns into four kinds of light: White light, red light, yellow light, and black light. Likewise, the light in all the sefirot is simple Divinity and unity, from the beginning of Atzilut until the end of Asiya.
The reason for the division into ten sefirot: Ḥokhma, Bina, Tiferet, and Malkhut, is due to the vessels that are called Ḥokhma, Bina, Tiferet, and Malkhut. For each vessel is like a refined partition, through which the Divine light passes to the recipients, and therefore each vessel is considered as turning the light into a different color. Thus, the vessel of Ḥokhma in the world of Atzilut transports white (meaning colorless) light, since the vessel of Atzilut is like the light itself, and the Divine light does not undergo any change on account of Atzilut when it passes through it.
This is the mystical meaning of the statement in the Zohar (Tikkunei Zohar, Introduction 3b) concerning the world of Atzilut, that “He, His life-force, and His attributes are one” (as explained below).6As the divine light descends to lower levels of existence, it remains intrinsically unchanged. However, the vessels that contain it at each level make it appear to change, relative to the created beings.
The point here is that since Atzilut is a very lofty level, to us it is indistinguishable from the divine essence of the Creator. At this level, the concept of separation does not really exist in a meaningful way, so this level is viewed as "one" with the Creator. Accordingly, the light of Atzilut is considered white light. This is not the case for the vessels of the worlds of Beria, Yetzira, and Asiya, where the light already receives on their account a certain change and dullness, when it passes through them to the recipients.
That is, the red light represents Bina, which is Beria; the yellow light, which is like the light of the sun, is for Tiferet, which is the world of Yetzira; and the black light is for the sefira of Malkhut, which is the world of Asiya.