the manual says mmm-default-submode-face sets color for all submodes the same ?
this would be Low coloring or
(setq mmm-submode-decoration-level 1)
so it might be you want
(setq mmm-submode-decoration-level 0)
which turns off submode background in mmm (i think)
also possibly use
(set-face-background 'mmm-default-submode-face nil)
or leave that line out alltogether
it's real easy in emacs to have one color setting stomp on the other.
if you can stop mmm
you might be able to set background for mode like normal
try this just as another blind try on my part
(this works here for naked non-mmm modes)
Code:
(add-hook 'visual-basic-mode-user-hook
'(lambda ()
(set-background-color "grey15")))