I recently bought the Saturn V and placed it on my windowsill where the rocket is exposed to the direct impact of sunbeams every day. Having encountered several yellowing effects on older white parts I wonder if this will speed up this unwanted process of discoloring?
Absolutely it will. The yellowing is caused by exposure to UV light, which it will get more of with direct sunlight shining on it. If it has to be in the window you could look at some glass coverings that block some of the UV, or find a home for it else where under electric light.