private void radGridView1_CellEditorInitialized(object sender, Telerik.WinControls.UI.GridViewCellEventArgs e)
{
    GridColorPickerEditor colorEditor = e.ActiveEditor as GridColorPickerEditor;
    if (colorEditor!=null)
    {
        GridColorPickerElement colorPicker = colorEditor.EditorElement  as GridColorPickerElement;
        colorPicker.ReadOnly = true;
    }
}

Please refer to the attached gif file.

Workaround:
private void radGridView1_EditorRequired(object sender, EditorRequiredEventArgs e)
{
    if (e.EditorType == typeof(GridColorPickerEditor))
    {
        e.Editor = new GridColorPickerEditor();
    }
}