I think the Belgian ML 160W, have the arctube at the top and the filament at the bottom, so they are intended to be burned base-up. Otherwise, the arctube wouldn't reach to full temperature and the filament would remain overdriven, resulting in a short life and hence the warmer colour.

@dor123: That explains the slow warmup when I ran it base down, took like 4 minutes to get to near full brightness - compared to the less than 2 mins warmup when running it base up