I'm not sure what these red and yellow stamps are that you're talking about. I have M+S tires and I don't have any colored stamps. Nowadays, most if not all "all-seasons" are rated M+S. "Snow tires" are something else beyond M+S and are only good for winter driving (marked with a mountain and snowflake). It's confusing because the Caltrans regulations refer to "snow tires" but they define "snow tires" as including merely M+S. So the regs do not actually require true "snow tires." Typical M+S all-seasons are adequate.
That said, I used to put chains on my WRX when it had summer tires. At one of the chain checkpoints, the guy waved me by when he saw that I had a subaru but at the last second he told me to stop. He asked why the hell I had chains on a Subaru. I told him it's because I had summer tires and he laughed and said get the hell outta here.
In doing some more research, I've found that the red and yellow dots are balance points indicating the high and low points on the tire, for the benefit of the installer -- they indicate where to balance the tire. This is bizarre, because in California we were told something totally different, and everything backed it up...there were no standard/small cars with the dots. We only saw them on larger SUVs and 4x4's.
I just did some Googling, and a couple of different sources said the same thing...the dots are reference points for the balance of the tire.
Now I wonder about the last shitty tires I bought. I asked for OEM tires, because the were awesome (Bridgestones). He sold me on some Hankooks, which were much cheaper. They SUCKED. And they didn't have the dots I was told about while in Tahoe. I replaced them with some new tires last Novermber, and the new ones have the dots (the Hankooks didn't). Hmm. So what is the real story? I need some more research!
Sorry for providing wrong info, but it was "real" at the time!
I'll update, and I look forward to tire geeks telling me what the deal with the dots is.