(note that i don't have a device, so i haven't seen the colors)
16 bits can encode 65536 colors. That's actually pretty little and is immediately obvious - just set your desktop color to 16 bits and feel the pain. From the films i've seen of the device, 16 bit color is unlikely. 18 bits would be extremely weird, since 18 makes no sense (multiples of 8 are preferred for technical reasons)
24 bits may sound weak compared to the 32 windows boasts, but as my linux desktop can confirm, there's actually no difference. My money is on 24 bit color for the n900
I wouldn't blame the makers of angry birds if they cut some corners to more easily port it to maemo. How much would you invest in porting your app to, say ... solaris (i.e. a niche, not very used os)? I think that's where the problems come from: the producers not optimizing the app very much
Here's a better benchmark: upload the same movie to both devices (assuming ye olde iphone will let you do that ). Then run is side-by-side and see