Honestly, with the exception of your color requirement, it sounds like you're describing an iPhone. I was, for the longest time, completely against them but I've since acquired an iPhone 4s and I won't go back to Android at this point. And don't get me wrong - I loved my Android phones (I had a G1 and a G2, both T-Mobile) but I'm far happier with the iPhone now.
I'll go point by point but I feel like it's not necessary really:
It has to be easy to use. - Check - it doesn't get much simpler. For this simplicity, you do give up a lot of your ability to customize but even that comes with a benefit, explained next.
It has to not stutter randomly. - Check - I have found iOS to be significantly more stable than Android. Of course, no OS is perfect and there will inevitably be a crash or something similar but it seems to happen extremely infrequently, especially when compared to Android.
It has to work well even under stress. - I'm not entirely sure what you mean by this. If you mean it needs to be capable of multi-tasking, the newer generations of the iPhone handle this in stride. If you mean they need to be physically tough, you may want to look elsewhere or invest in a good case (which could help you with your color requirement).
It has to have good battery life (at least 4 hours of web browsing (seriously my Galaxy S dies before the 3DS)). - Check - I know there are reports of poor battery life and they're true if you expect a week on a single charge but I use my phone a lot, have location services enabled and push notifications for all of my accounts and I still last well beyond the day. As it stands, I'm just under 40% battery and it's been off the charger for 16.5 hours.
It has to have good audio quality (I do listen to music on the go a lot) - It has an iPod built in. Enough said? Of course, audio quality is going to also be dependent on the headphones you use but the base audio quality is good.
It CANNOT have a PenTile display (I hate how the text looks on these kind of displays.) - Check - Retina display. Definitely enough said there.
I don't want a black or white phone. Had it with booooooring colours! - Short of a colored case, this is the only area where it definitely does not fit your expectations.
Unfortunately, I've been out of the loop with Android recently so I can't recommend anything you likely don't already know but as you want to avoid it for now anyway, that shouldn't be a problem. As for Windows Phones, I haven't had any personal experience with them but I have heard good things about Windows Phone 7 so it may be worth looking into as well.