You need to find out which highlight groups it's using and then modify them. I believe the drop-down menu is using Pmenu and PmenuSel, so you can tweak this with:
:highlight Pmenu guifg=#HEXCODE guibg=#HEXCODE gui=DECORATION ctermfg=COLOR ctermbg=COLOR cterm=DECORATION
HEXCODE can be a hex color code and is used in gvim (or macvim or similar).
COLOR is used in console mode (where colors are supported). You can look up a list of valid colors in :help cterm-colors (but it only lists colors for 16-color terminals, there may be more).
DECORATION is a decoration style (bold, underline, undercurl, etc).
For more more information, :help highlight
Note, changing this will change the Pmenu and PmenuSel globally, so anything else in vim that uses those two highlight groups will also have their colors effected.