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When exporting a dwg it originally has colors according to parts created in AECOsim Building Designer, where colors are often applied by layer. Once the model is exported to dwg it shows the correct colours, but if updated according to dgnlib it will now display wrong colour with no easy way of reverting this back. I believe this is because the colour table is different in dwg, and if the software wants to reapply the color associated to a certain number it will be different if the colour table has changed. How can I get the colors to display as intended if an exported dwg-file is unintentionally updated?
Your assessment of the issue is correct. AutoCAD DWG file only use one color table. When we convert from DGN to DWG, we match the colors based on the RGB color value. This causes the colors to look the same but the color numbers will change. Example: the standard AECOsim Building Designer color 1 is blue, when saving to DWG the color remains blue but changes to color number 5.
AECOsim uses color numbers in the design files and in DGN Library files. So, when we update the levels color, we update it to the color number used in the DGN Library file.
The following is the current workaround:
For projects that will be converted to DWG, use the AutoCAD color table, from the start, in the DGN drawing files. We deliver the AutoCAD color table with AECOsim. The name of the AutoCAD color table is ACADCOLOR.TBL. Also make a copy of the DGN Library file and use the AutoCAD color table in that file also. Set the colors in this DGN Library file based on the AutoCAD color numbers. For all projects that will be converted to AutoCAD, set the project to use this AutoCAD based DGN Library file and use the ACADCOLOR.TBL color table for all the drawings.
The only way to prevent colors from changing is to use the same color table in the DGN files, the DWG files and in the DGN Library.