When calling XinfobarProvider.appendInfobar or updateInfobar, you specify an infobarType parameter. The color of the infobar is determined entirely by that infobarType enum. It would be a nice touch if we could add a method to the XInfobarProvider that allows you to apply a custom color to an infobar.
Don't see a reason the change the color. Red, yellow, green and blue were carefully picked for bug 105829 in respect to the type of information and there is no need for a pink warning, for instance.