Check the FAQ here: http://web-mode.org/

It might be a problem with othere modes: "Syntax coloring does not work

Do not enable rainbow-mode or whitespace-mode with web-mode (see above). Moreover, some users might report issues when using themes that not play nicely with web-mode. Tip: when using emacs in a terminal, always try to enable 256 colors compatibility (xterm-256color)."

Update: Fixed some typos.