Example to set .bk  to highlight as a bash file
 

vim /etc/vimrc

Syntax on 
au BufNewFile,BufRead *.bk set filetype=bash


Create your own syntax on a custom extension (raw) in this example
 

Create the syntax file ~/.vim/syntax/raw.vim
 
" Vim syntax file
" Language: jlweber raw files
" Maintainer: Jean-Louis Girard
" Latest Revision: 2018-08-16

if exists("b:current_syntax")
  finish
endif

" Regions
" remove the escapes for code
syn region jlcode start='<\code>' end='<\/code>'
syn region jlh1 start='<h1>' end='</h1>'
syn region jlh2 start='<h2>' end='</h2>'
syn region jlh3 start='<h3>' end='</h3>'
 

Create the filetype for vim ~/.vim/ftdetect/raw.vim
 
autocmd BufRead,BufNewFile *.raw set filetype=raw


Add the desired colors to the region matches in .vimrc

set t_Co=256
highlight jlcode ctermfg=39
highlight jlh1 ctermfg=172
highlight jlh2 ctermfg=178
highlight jlh3 ctermfg=184