If you go to Stadia.com in the Chrome browser, then click on the settings dropdown menu at the top right of the browser, you will see "Open in Stadia." Clicking on that will open the Stadia app, and it works.
However, playing in Chrome is much smoother than playing in the app. The app speeds up and slows down a lot. I hope that the performance in the app can be improved to match the performance in the Chrome browser.
I have the same issue, a lot of crazy color artefacts on my Surface Pro X. Nothing helped me so far. I tried different Browsers (Edge, Chrome), the apps and incognito mode. I also tried to turn off the HDR mode but nothing lead me to an nice graphic representation.
You can workaround this disabling hardware decoding in chrome://flags #disable-accelerated-video-decode or forcing h264 (see stadia+ extension)
Still a fix would be great...