hello, same here Polaris doesn't work. i got pi-top last week, but no luck with PiTopOS... raspbian stretch full image works fine.
To run a Raspberry Pi 3B+ you need to upgrade pi-top os to the newest version. Otherwise the 3B+ won't boot. The 3B+ is only supported since Raspbian Stretch and the delivery version of pi-top os polaris is based on Raspbian Jessie. This only works up to the Raspberry Pi 3B, not plus.
Apologies for not sharing feedback on the 'rainbow screen' issue you've been experiencing. As kindly stated by Alexander, you will need to update your SD card with the latest pi-top OS preferably via the pi-top website to work with the Raspberry Pi 3B+.
I am having the same issue on a pi-top v2 with a R-Pi 3 B+. Actually I have tested two new R-Pi 3 with two different original micro SD cards inside the pi-top and standalone with an alternative power supply and a HD display attached. All I get is the rainbow screen.
I understand that debugging problems is part of the learning experience and therefor somewhat part of the whole pi-top idea, but this is this is a terrible customer experience. I am shocked to see that seemingly no moderator from the pi-top team has reacted to the previous posts after 5 and 7 month respectively.
Help! Please ... anyone?
Can you test the R-Pi 3 B+ outside of the pi-top to confirm if it is working on its own?