Vim: How to change the current item highlight color when substituting

The highlight group you want is "IncSearch" even if you don't have the the 'incsearch' option enabled—it is also used for the current substitution when confirmation is enabled.

See ":help hl-IncSearch".

When inside vim you can use the command mode:
:highlight IncSearch guibg=green ctermbg=green term=underline

To have it always, put this into your .vimrc:
highlight IncSearch guibg=green ctermbg=green term=underline