Am having to install covers to several different types of electrical connections boxes in my house. Some are round, blue ones and some are a grayish tan with corners rounded. None of them have cover screws in them, so I am having to go buy screws to attach the covers. What type and size of screws are need for each type of connection box?
Most hardware stores I have been to had a specific electrical screw section in their fastener area. They should basically be labelled as electrical box cover screws, and you can find them in all kinds of versions: long/short, metal/plastic, and different finish colors.
I maintain a supply of #6-32, #8-32 and #10-32 (which is a fine thread) in my electrical screw bin.
Code requires a -32 thread or finer for grounding screws, because a standard steel junction box has a metal thickness such that -32 will give enough thread engagement to be secure. -40 would be allowed; -24 would not. By coincidence, NC (National Coarse) thread sizes are 6-32 and 8-32, and NF (National Fine) is 10-32. And that is why this tool exists.
I will attempt to answer but, Could you provide photos, your description almost sounds like wire nuts both made in tan and blue. I think you are talking about cutthler hammer panels the gray/tan but there are a large number of different manufactures of the blue ish covers.
load center cover screws are not standard screws. They have a larger head, some have a fine pitch like Siemens ects2 or Square D S106 then almost wood screw look for Eaton LCCSCS. These are closer to #10 size with a large head #2 square drive or flat blade screwdriver and the first 1/8" is not threaded to get the screw in the hole. I know quite a few electrical inspectors and home inspectors that look for the correct screws this is why I provided the part numbers of the main ones I keep spares of.