The iconic 425/17 is now believed to have been delivered in clear-doped natural linen upper surfaces and doped pale blue below.
Tell-tale sign of the blue undersurfaces practice in DR.I photographs is that the colour was continued onto the bottom fuselage longeron line and bordered the horizontal stabiliser and elevators.
The fact that this is visible indicates to me that blue is not present as an overall base colour on production DR.I’s.