As mentioned in comments, I am not aware of any out-of-the-box way for ArcMap to symbolize layers using RGB values stored in an attribute table, or using RGB values in Excel joined onto an attribute table.
However, there is an existing ArcGIS Idea to Set symbol color from RGB values in attribute table so I recommend that you add your vote to that.
Note that the above ArcGIS Idea only covers:
The ability to set the fill colour for a symbol from RGB values stored
in the attribute table of a shapefile/feature class.
So if you are wanting to do this for points, polylines or outlines of polygons then you may want to submit additional or more comprehensive ArcGIS Ideas.
Alternatively, if you use ArcGIS Pro, you can use a few lines of ArcPy/Python to calculate a hexadecimal color field from your RGB values and then use attribute driven symbology.
For more details see Using RGB values stored in field of feature class to apply colour component for ArcGIS Pro symbology?