The straightforward method of creating a color image is to combine red, green, and blue filtered images together. This works well because the human eye works exactly the same way; there are individual red, green, and blue receptors (cone cells) in the retina.
When working with low light levels, it can be difficult to achieve sufficient exposure time in all three color bands to produce a clean, low-noise image. The LRGB technique is a shortcut that can be used to reduce the total exposure time required, by adding a Luminance frame to the three standard color planes.
For the LRGB technique, you capture an unfiltered image set with a very long exposure time. To get the color information, separate red, green, and blue sequences are taken with a shorter total exposure time. The three planes are combined together to produce a color image, and then the luminance of that composite is replaced with that from the long unfiltered exposure set. This LRGB technique can be done using the Stack or Combine Color command.
This technique works well because the human eye is far more sensitive to luminance changes than it is to color changes. This technique has been used for many years to reduce the bandwidth required for transmitting color television. The black-and-white component of the television signal is transmitted with full bandwidth, and the color components are transmitted at about 1/3 bandwidth. When the image is reassembled on the TV screen, the eye simply does not notice the lower resolution of the color components.