I think there may be 2 possible solutions.
1) Make sure you are referencing the namespace correctly at the top:
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imports ESRI.ArcGIS.Display;
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In your code, you can simply use IRgbColor.
2) Or you can not have the imports statement and just put when you are creating a pointer to the class.
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ESRI.ArcGIS.Display.IRgbColor
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Hope this helps.