Color codes are not all they're cracked up to be, IME.
But, yes, upon booting up, it is normal for the Amiga to go from black, to a dark grey once the video syncs and the machine starts to boot.
Look at the CPU slot under magnification (as well as the connector on the 3640 itself) and make sure none of the little pins inside have gotten bent.
Are you getting any activity on the _DS and _AS lines on Buster?
