Architectural drawings take a lot of work to make more presentable . I usually follow the method of plotting from autocad as a Tif file -> open in gimp -> erase the background -> save as an xcf file in gimp -> add a background colour -> add more colour to areas above the background but below the lines -> some shadows -> text . Then I save a copy as png . So at any moment I have the same filename with tif , xcf and png format . Keeping three different formats allows me to make changes as I go along .
Erase the background with choose by colour tool and colour to alpha . Add a layer call it base and drag it to the bottom . You can change the colour easily once everything is on different layers.
Create another layer for fills and move it above the base layer and below the linework . Select areas and fill colours . Select by colour tool again and drop shadows .
Add annotation and graphics. You are done when you think you are .