Truecolor works just out of the box within the terminal emulator of my choice - st by suckless. Neovim looks perfect. However, I've recently noticed that :set termguicolors breaks the colors in Vim. I've reproduced the issue without .vimrc by running :set termguicolors and then :hi! LineNr ctermfg=red ctermbg=red guifg=red guibg=red or any other highlight group. With truecolor ready colorschemes, such as solarized8 or gotham256, I get only white text over my terminal's default background.

:help

VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Aug 21 2016 12:29:10)
Included patches: 1-2232
Compiled by Arch Linux
Huge version without GUI. Features included (+) or not (-):
+acl +dialog_con +job +mouse_sgr +rightleft +user_commands
+arabic +diff +jumplist -mouse_sysmouse +ruby +vertsplit
+autocmd +digraphs +keymap +mouse_urxvt +scrollbind +virtualedit
-balloon_eval -dnd +lambda +mouse_xterm +signs +visual
-browse -ebcdic +langmap +multi_byte +smartindent +visualextra
++builtin_terms +emacs_tags +libcall +multi_lang +startuptime +viminfo
+byte_offset +eval +linebreak -mzscheme +statusline +vreplace
+channel +ex_extra +lispindent +netbeans_intg -sun_workshop +wildignore
+cindent +extra_search +listcmds +num64 +syntax +wildmenu
+clientserver +farsi +localmap +packages +tag_binary +windows
+clipboard +file_in_path +lua +path_extra +tag_old_static +writebackup
+cmdline_compl +find_in_path +menu +perl -tag_any_white +X11
+cmdline_hist +float +mksession +persistent_undo -tcl +xfontset
+cmdline_info +folding +modify_fname +postscript +termguicolors -xim
+comments -footer +mouse +printer +terminfo -xpm
+conceal +fork() -mouseshape +profile +termresponse +xsmp_interact
+cryptv +gettext +mouse_dec +python +textobjects +xterm_clipboard
+cscope -hangul_input +mouse_gpm -python3 +timers -xterm_save
+cursorbind +iconv -mouse_jsbterm +quickfix +title
+cursorshape +insert_expand +mouse_netterm +reltime -toolbar