Dean,
I'm no expert either however I have some excellent Reference Publications and thought I'd briefly rienforce the replies you've had. 425/17 left the factory all red from "head to toe" with the (curved outline) Eiserne Kreuz's applied. By March 1918 the crosses had been changed to the "Balkenkreuze format (straight design like the Iron Crosses of WW2) and a white rudder was already on or was applied with the cross change.(I believe the latter would suit logic.) Sometime in the first two weeks of April 1918 the crosses were further "narrowed" to comply with directives. Thus to model 425/17 for 21 April 1918 a white rudder with the narrow Balkenkreuse insignias is correct. The excellent Osprey book, Fokker DR1 Aces of World War 1 confirms this as well. The Baron had several other "spares" he flew as you stated, personally I prefer several of them to "17" from a modelling perspective.
The above is mainly from AE Ferko's excellent "Richthofen" book from Albatros Publications, the folks who do Windsock Datafiles. The editor is an acknowleged expert, modeller extrodinaire, and well known in our little community.
Hope this helps, don't hesitate to ask if you need more. I enjoy the exchange of info and perspectives on our hobby as much as the workbench part; shows in that I am doing more on line these days then building!?
Cheers,
Lance