I have tried to make one menu a specific colour different from the other menu items.

I first used

#ext-gen9{
background-color:#C00 !important;
background-image: url(../images/red.png);
background-repeat: repeat-x;
}/*

Which worked, however when menus were changed this number eg9 changed and there was no control.

I thought about using the Module Class Suffix, but this does not appear, then I thought to use the Menu Image facility and this did not appear either.

Can you advise the best way to colour a menu differently from the rest with a little more control.

Hello,

Each menu item has unique CSS class name based on menu item ID. You can read about it here.

Regards,
ARI Soft