hmmm,
since a sodium lamp only emits 590nm wavelengths which colour paper is supposed not to be sensitive to, the cover serves the purpose of diffusing the light. It doesn't need to act as a filter by being coloured. Hence The cover is white. The brochure images show it with lamp on which makes it look/emit faint amber.
Of course the duka 10 could be different to my duka 50. And if the Duka 10 doesn't have a sodium lamp in it, then yes the cover should be light amber.