For the 3mf file this makes no sense. These are unsliced and it is not predictable in which colors they will be used later. You would then need to define all colors during upload and decide then which you want - very impractical and time consuming.
For the gcode files we have some plans. We want to introduce color marker so that when they are present, the renderer will use the selected colors from slicing. This will first work with our slicers, but if other slicers add color information readable for the extruders we then want to detect them as well. If none are present, they will of course be rendered with default colors again.
For the upcoming live 3d preview we will most likely use material manager colors when that module is finished, so it shows with colors currently loaded then and would also allow showing a comparison before print to see if colors are ok.