When adding devices to your FortiManager using scripts, you can set the color of the object.
I did not find any documentation for what color ID matches the actual color in the FortiManager. So I created this script and ran it in order to create objects in all the different colors
config firewall address
set subnet 10.10.10.10 255.255.255.255
set color 1
Related: Adding firewall objects using script in FortiManager