This is how ilvl 1000 looks like in my bags.
https://i.imgur.com/rxoqnVy.png

https://github.com/AdiAddons/AdiBags/blob/master/modules/ItemLevel.lua#L139
If I comment this line out, it temporary works. (Text color is default white)

Does replacing it with the following correct it?

			text:SetTextColor(colorSchemes[settings.colorScheme](tonumber(level), quality, reqLevel, (loc ~= "")))

No. Debugging help:

			if(level == 1000) then
				print(type(level)) -- number
				print(type(quality)) --number
				print(type(reqLevel)) --number
				print(type(loc)) --string
			end

I'am guessing the colorSchemes table if not suited for that ilvl. But I cannot for the world find out whats "in" that table.

I have https://www.curseforge.com/wow/addons/bugsack and https://www.curseforge.com/wow/addons/bug-grabber installed, but I dont know how to trigger their windows to see the variables.

https://github.com/AdiAddons/AdiBags/blob/master/modules/ItemLevel.lua#L216

It looks like the 999 on that line is intended to be higher than any obtainable item level. Changing it to 9999 seems like a reasonable short term fix.

If the next expansion really does an item level squish then this color scheme will probably need some work.

This is no longer an issue (item levels have been squished).