It is widely accepted that the ASET image of any MRB should contain a mix of green, red, and blue. The lack of any one color means that there is something wrong with the photography setup, or maybe even with the stone itself.
How the Idealscope and ASET Work
Simply put, these are mechanical devices, which have colored parts that act as filters. The incident light on the stone picks up the color of the filter, and this is seen in the reflected light when you view it through a scope lens. Understanding the working is easier if you have one of the devices in hand.
Is More Red Better than More Green?
In fancy cut diamonds, more green is often preferable over more red. If most of the stone is green in an ASET image, it means that more of the light is being sent back from closer to the fringes of the stone.