I haven't looked, but I'd guess that:
#slider {background-color: #f00; /* or whatever colour you want */ }
would work. Assuming that the #slider
element is what I think it is.
Edited:
Sorry for taking so long on this, having looked at the jQueryUI's demos (particularly for the color-picker: http://jqueryui.com/demos/slider/#colorpicker) it seems that the background-color
property seems to be applied via additional classes applied to the element.
Changing the class from ui-slider-range
to ui-slider-range-max
seemed promising, but achieved nothing (except from, perhaps, the obvious flipping the coloured area around).
This is weird.