Why do Vim colors look different inside and outside of tmux?

Environment:

Fedora 25 (4.9.12-200.fc25.x86_64)
GNOME Terminal 3.22.1 Using VTE version 0.46.1 +GNUTLS
VIM – Vi IMproved 8.0 (2016 Sep 12, compiled Feb 22 2017 16:26:11)
tmux 2.2

I recently started using tmux and have observed that the colors within Vim change depending on whether I’m running inside or outside of tmux. Below are screenshots of Vim outside (left) and inside (right) of tmux while viewing a Git diff:

My TERM variable is

Outside tmux: xterm-256color
Inside tmux: screen-256color

Vim reports these terminal types as expected (via :set term?):

Outside tmux: term=xterm-256color
Inside tmux: term=screen-256color

Vim also reports both instances are running in 256-color mode (via :set t_Co?):

Outside tmux: t_Co=256
Inside tmux: t_Co=256

There are many similar questions out there regarding getting Vim to run in 256-color mode inside tmux (the best answer I found is here), but I don’t think that’s my problem given the above information.

I can duplicate the problem outside of tmux if I run Vim with the terminal type set to screen-256color:

$ TERM=screen-256color vim

So that makes me believe there’s simply some difference between the xterm-256color and screen-256color terminal capabilities that causes the difference in color. Which leads to the question posed in the title: what specifically in the terminal capabilities causes the Vim colors to be different? I see the differences between running :set termcap inside and outside of tmux, but I’m curious as to which variables actually cause the difference in behavior.

Independent of the previous question, is it possible to have the Vim colors be consistent when running inside or outside of tmux? Some things I’ve tried include:

Explicitly setting the default terminal tmux uses in ~/.tmux.conf to various values (some against the advice of the tmux FAQ):

    set -g default-terminal "screen-256color"
    set -g default-terminal "xterm-256color"
    set -g default-terminal "screen.xterm-256color"
    set -g default-terminal "tmux-256color"

Starting tmux using tmux -2.

In all cases, Vim continued to display different colors inside of tmux.

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Answers:

Method 1

Method 2

Method 3

Method 4

Method 5

Method 6

Answers:

Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.

Method 1

I had the similar issue before. Comments in blue in Vim were hard to read. In .tmux.conf I set this:

set -g default-terminal "screen-256color"

And in .vimrc:

set background=dark

Now it looks as follows and works both in Gnome Terminal and Cygwin:

Method 2

tmux doesn’t support the terminfo capability bce (back color erase), which vim checks for, to decide whether to use its “default color” scheme.

That characteristic of tmux has been mentioned a few times –

Method 3

Thanks to @egmont’s analysis of what colors Vim was outputting when TERM=screen-256color, I was inspired to look at the color scheme Vim is using in the two scenarios.

Vim reports it is using the default color scheme in both cases. I thought that odd because the default color scheme on Fedora 25 (/usr/share/vim/vim80/colors/default.vim) doesn’t appear to match the colors I actually see when TERM=xterm-256color. If I explicitly set the color scheme using :colorscheme default when TERM=xterm-256color, Vim’s appearance changes to that when TERM=screen-256color. To get the colors back to what they were when I first start Vim, I had to use the ron color scheme. Progress!

I found an Ask Ubuntu answer that suggests that when Vim reports it is using the default color scheme, it doesn’t necessarily mean default.vim but rather some theme-specific color scheme. As the answer points out, a dark theme (which I am using) corresponds to the ron color scheme, just as I discovered above. (Even though this post is with respect to Ubuntu, I’m assuming the OP was using GNOME.)

I also found another question that seems to describe the same problem I’m having. I came across it while searching before I posted this question, but, for some reason, the color scheme didn’t strike me as being relevant.

I ended up doing what @LapshinDmitry did in his answer and explicitly set colorscheme ron in my ~/.vimrc file. Now, whether I start Vim inside or outside of tmux, the colors appear the same. The only drawback is if I ever change my desktop theme from a dark flavor to a light flavor, Vim won’t automatically switch to the “default” light theme color scheme, which is apparently peachpuff. I can live with that, as I’m unlikely to ever change my theme.

I’m not going to accept this answer because I consider explicitly setting the color scheme in my ~/.vimrc a workaround rather than the solution. If someone can explain why Vim loads a different “default” color scheme depending on the value of TERM, I’ll be happy to accept that answer, as I’m more interested in understanding the root cause. I suspect it has something to do with how Vim interprets the terminal capabilities between the two terminfo files.

Method 4

My TERM variable is
Outside tmux: xterm-256color
Inside tmux: screen-256color

That’s correct and working for me. Try it with a different terminal (I use urxvt) to see if Gnome Terminal is the problem.

Also check COLORTERM (mine is set to rxvt) and unset TERMCAP.

Method 5

Thanks for all the answers. I have some new findings.

In VIM, there are default color schemes and a color scheme named ‘default’. Confusing, huh? The default color schemes are ‘ron’ for dark background, and ‘default’ otherwise. Checking
https://github.com/vim/vim/blob/master/runtime/colors/default.vim, set bg& is causing the trouble.
Only dark background is affected by this problem because the background is mistaken as light. Query :set background? to find out.
iTerm2 works with no issue when TERM=screen-256color. But it still does not work when you run tmux in iTerm2. The reason is iTerm use COLORFGBG, which vim can use to determine bg. This explains many magic behaviours. iTerm2 sometimes adds COLORFGBG, but sometimes not.

workaround:

Explicitly set colorscheme, e.g. colorscheme ron
set background=dark
Passing query of bg to outer terminal by

if exists("$TMUX")
    let &t_RB = "ePtmux;ee]11;?07e\"
endif

So far, this is the method I’m using. The best would be tmux to implement support of osc 11 sequences and Vim recognize tmux and set t_RB accordingly.

Method 6

For me I have to add below into the .tmux.conf to get the RGB color and eliminate the color difference.

set -ag terminal-overrides ",xterm-256color:RGB"
set  -g default-terminal "tmux-256color"

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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