In a laser printer for ONLY black ink the image to be printed is composed of an array of very tiny black (toner) dots. The printing process has several steps.
1. The entire surface of a rotating specially-coated drum is charged up with a static charge.
2. As the drum rotates it is then exposed to light from a laser (in Brother's printers, often this is from LED's rather than a laser, does not matter) where the final image is supposed to be white. This removes the static charge at those points, leaving charges still on the drum in the exact places for the black toner parts.
3. The drum passes a closely-spaced source of toner particles that are charged the opposite way, so they are attracted to the drum charges, creating the toner image on the drum surface.
4. The turning drum then makes actual contact with the paper surface, transferring all the toner particles to the paper.
5. Final step for the turning drum is a soft brush to clean off any toner that was not transferred, and then back to the charging position.
6. For the paper, its next step is to pass under a very hot wire called the fuser. The heat melts the thin coating of adhesive on the toner particles, making them stick permanently to the paper surface. The paper continues on to the discharge chute.
NOTE that this process can create and print an image in only ONE toner colour. Once the drum has been though the first three steps you cannot re-charge the white areas while maintaining the parts that already have some toner particles attached, re-expose those to a different light pattern, and deposit on those areas a new toner of a different colour.
To print in "full colour" this same process is followed for FOUR different toner colours. EACH toner colour can be done by its own separate drum system, and the image each drum creates is ONLY what is required from that colour as part of the overall array of toner particle dots. But the process is different between steps 3 and 4 in a colour laser printer. There is a special transfer belt running horizontally in the printer under the four drum units. The toner from EACH unit is transferred to this belt and NOT directly to the paper, so that the entire image of toner dots of all four colours are accumulated on the belt. Then the BELT is pressed against the PAPER to transfer all the toners (similar to step 4 above) , followed by passing the paper under the fuser wire and out. As the belt continues its travel, it also (like each individual toner drum) gets brushed clean before its next cycle.
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