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
edit “test-color-1”
set subnet 10.10.10.10 255.255.255.255
set color 1
next
end

Related: Adding firewall objects using script in FortiManager