Hi,
I want to change autocomplete menu background color of neovim.please find my attachment.
You can use the highlight command with the highlight groups Pmenu, PmenuSbar and PmenuSel. Specifically, the background can be changed with the Pmenu group like this:
hi Pmenu ctermbg=10 ctermfg=15 guibg=#ffffff guifg=#000000
Above I put both cterm and gui settings, but the gui part can only be used by setting termguicolors set termguicolors, somewhere in your init.vim.
You can read more on highlighting on :help hi.