You can assign the reflectance value at the time of importing your image or creating your RGB.
Go to Home Manage Materials.
From the drop down, select if it is a User Image or an RGB colour you wish to create.
For a User Image, select to Add (This Design) or Add (Any Design) and point the file explorer to the image you wish to import (*.bmp, *.jpg, *.jpeg, *.png).
For a User Image, select to Add (This Design) or Add (Any Design) and give it a Name.
Select the colour:
By clicking Other... on the standard drop down for colours, the window below will launch, Standard as shown left and Custom as shown right.
Both the Standard and Custom have the pipette (Select..) tool.
Once you have set your colour or imported your image in 2020 Fusion version 8.6, you are given the option of Reflectance in here and are given 12 Options as follows:
Below is an example of 4 different reflectance options applied to the same raw image.
The Glow option creates an ambient glow such as what you would expect from a TV screen, laptop or mobile phone. With this reflectance, you can increase or decrease the intensity of the glow from the Items Lights and by using the -|+ slider.