I just spent quite a lot of time figuring out how to change popup background colors. I use coc.vim so I incorrectly thought coc was setting the color. Then I accidentally typed :hi and it displayed ALL my colors, and there discovered my Pmenu was set to magenta. I don’t know how it got set to that, but to change it all I had to add in my vimrc was highlight Pmenu guibg=#808080 or whatever color you want. You may need to set ctermfg, ctermbg, gui, guisp, or cterm depending on if you’re using the gui or not, and if you’re trying to set a foreground or a background.
How to Change Vim Colors – Highlights, Backgrounds, Foregrounds, etc.
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