The official documentation at DataBinder.Eval Method (Object, String) is quite clear about this:

Expression: The navigation path from the container object to the public property value to be placed in the bound control property. This must be a string of property or field names separated by periods, such as Tables[0].DefaultView.[0].Price in C# or Tables(0).DefaultView.(0).Price in Visual Basic

So the second expression should work fine if Colours has an indexer (this[int index]) property defined.