How to keep meringue white while baking?
First, double check that your oven temps are accurate with a thermometer that you can place in the oven. 

After that, two things I would try..


Rack Positioning 


I've baked meringues at similar temps without browning. It could be the position of the rack within the oven? If it was mainly the top that browned, move lower. If it was mainly the bottom that browned, move higher. 


Turn Off the Oven


The other thing you could try is turning the oven off for the second stage of baking. I've baked meringues at 200 degrees F for an hour before, then I turn off the oven, prop the door open for about 5 minutes to cool, then close the door for a few hours. 

Hope one of these give you the results you're looking for.