The type 'Color' is defined in an assembly that is not refer

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i've got a problem with ios project.....

tChart.Header.Font.Color= Color.Blue;

Error: The type 'Color' is defined in an assembly that is not referenced.
You must add a referenced.You must add a reference to assembly 'OpenTK-1.0,Version=0.0.0.0, Culture=neutral......'