Not working with ft=tex
Describe the bug
In TeX documents, the plugin is not working inside
\begin{document}
...
\end{document
To Reproduce
init.vim/vimrc:
call plug#begin('~/.local/share/nvim/plugged')
Plug 'luochen1990/rainbow'
call plug#end()
set nocompatible
let g:rainbow_active = 1
let g:rainbow_conf = {
\ 'separately': {
\ '*': {},
\ 'tex': {
\ 'parentheses': ['start=/(/ end=/)/', 'start=/\[/ end=/\]/', 'start=/{/ end=/}/'],
\ },
\ },
\}
Example document:
\documentclass[]{article}
{{{{}}}}
((((((()))))))
[[[[[[]]]]]]
\begin{document}
((((()))))
${{{{}}}}$
{{{{}}}}
[[[[[[]]]]]]
\end{document}
Expected behavior
For the colors to appear equally inside and outside the document environment.
Screenshots

Notes
It works for the same document when changing ft: set ft=text
Related issue? https://github.com/junegunn/rainbow_parentheses.vim/issues/10
Is there some special reason which makes you specify separately.tex.parentheses by yourself? Because in the default configuration, {} is not included in separately.tex.parentheses.
The curly braces are the most important to me, but the same problem is there without:
Here is an updated version: init.vim/vimrc:
call plug#begin('~/.local/share/nvim/plugged')
Plug 'luochen1990/rainbow'
call plug#end()
set nocompatible
let g:rainbow_active = 1

@mreppen did you ever figure this out? I'm experiencing the same problem.
@bisraelsen Unfortunately not. My understanding of the deeper workings of vim is too shallow to really try anything useful.
pushed a fixup, anybody help to make sure it is correct ?
Thanks, that gives a bit more color. However, in the "texMathZone?" regions it does not work:

Running echo join(reverse(map(synstack(line('.'), col('.')), 'synIDattr(v:val,"name")')),' ') gives texMathZoneX texDocZone inside $...$ and texMathZoneW texDocZone in \[...\]. There are apparently also versions with V and Y.
Manually adding these (and possibly other) cases works, but it would be more elegant if the containedin=texDocZone were sufficient. Do you think that is possible?
I'm not very familiar with tex's syntax, can you provide a list of containedin names along with a test file for what you want? or a pull request?
I guess that the basic idea is that math equations are contained either within single dollar signs $x=y^{z+2}+(z+0.5)$ or in double dollar signs $$x=y^{z+2}+(z+0.5)$$. There is a third way to enclose math that is shown above: \[ x=y^{z+2}+(z+0.5) \]
Basically anything within $ $, $$ $$, or \[ \] is mathematics and should have matching braces matched. For that matter, even the $ $ and \[ \] enclose things and I guess could also theoretically be colored for matches as well.
I will write a pull request with containedins for all delimiters I find relevant in TeX. It will need some testing on different documents, but I should find time one of the coming days.
There are some cases of curly braces that I cannot make work. For instance, in the case of this figure

It seems like the rainbow zones are not loaded for the first case regardless of what containedin=... I enter.
@luochen1990 Do you have any clue?
Is there a self-contained example which I can use to reproduce your problem?