I am not sure what exactly are the requirements of your scenario. In spite of this, I have tried with an enumeration and it works correctly.
I have replaced the colors array with the following:
comboCol.DataSource = Enum.GetValues(typeof(System.IO.FileAccess));
I have also removed setting DisplayMember and ValueMember properties. However if you continue to experience problems, it would be better to send us a sample application that demonstrates your scenario.
As to the color issue, you could access the RadComboBoxEditor as you attach to the RadGridView.CellEditorInitialized event. In the handler, you can get the value of the RadGridView.EditingElement property. However, I am afraid that setting different colors of the items in its dropdown window is not possible. Perhaps this could be done if you inherit RadComboBoxEditor and fill its Items collection with runtime created items instead of binding to an enumeration.
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the Telerik team