Not a huge Adenium expert or anything (I have a few), but subtle to not so subtle changes in flower color are a pretty common phenomenon in cacti as well. I also have an A. obesum hybrid with pure white flowers (it did not come with a cultivar name attached) that will start to develop slight to very distinct pink tips on the flower petal some times halfway down a flower cluster. I have another one that has deep red flowers, which can be a lot darker and at some times develop black fringes on the petal tips.
In cacti I have seen changes in light exposure and humidity be the cause (as suggested above), as well as changes in day and night time temperatures, I suspect there might also be changes in the soil/fertilizing/water that could come into play, excess or scarcity of certain vital nutrients. I realize it is frustrating that if you have a named cultivar that is supposed to be pure white and it is not, but it is not an extremely rare phenomenon. Not sure if this is something that would affect Adenium flowers, but in many cacti and succulents a sign of sun/temperature/water stress is that they will turn all sorts of pink-purplish-reddish colors.
I guess what I would do is try and determine what if any changes there have been in how I handled the plant in recent time compared to say previous year when you moved it from the green house to the outside, how were the outside conditions now compared to last year... etc.
