Ventsim will only apply the default colour if it cannot find a colour listed in the DXF file.
There are many variations of how CAD DXF files are put together. The program has been successfully tested with many types however you may have a format that is not currently fully compatible. Try saving the DXF file in an eariler DXF format (if possible) or send the file through to
admin@ventsim.com so I can have a look at it.
Craig