That is exactly the algorithm used by this new software, too – anything posted since the date/time of the last time you viewed messages is “new” (green icon); anything older is “old” (yellow/tan icons).
The only problem occurs when you log on, read some (but not ALL) new messages. Then, the next time you log on, even the ones you did not read the previous time are marked as “old” this time around. This is the same problem the sr20.org board suffered from as well.
One difference between the two is that, as you pointed out, the sr20.org board used cookies to save this state. For me, that meant that when I logged on at work vs. logging on at home, the “new” messages were not consistent. (i.e. first thing monday morning at work, a bunch of messages were marked “new” even though I’d read them at home over the weekend).
The new s/w stores the state in your account, so no matter where you log in from, it knows what’s new and old. For me personally, this works better. (Note that I realize it doesn’t work better for everybody – if you read at work and your spouse [for example] reads at home, you might PREFER the “new” vs “old” state to be kept per machine rather than per account).