How do I pick a custom colour for fills in impress. At the moment I just see a drop down list with no option to define a colour myself (I have specific branding colours which must be used)
Right click the object. Click Fill. Go to the tab nammed Color. There, you can add / modify / delete colors to be shown in the drop down list you mention.
By the way, you should modify the template (aka mask) and not every slide.