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

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:

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 -

• Reset background to transparent with tmux?
• Clear to end of line uses the wrong background color in tmux

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.