Hi Ciara,

Have you checked that all colored objects in your drawing has only standard AutoCAD colors with numbers from 1 to 255 (AutoCAD Color Index):
https://forums.autodesk.com/t5/autocad-for-mac-forum/autocad-2013-for-mac-setting-plot-styles/m-p/39...

If some of objects have True Color assigned to their color property, then Plot style table (CTB) has nothing to do with them, they will always print in color.