Just trying to install osmc from scratch on my new B+ and I’m just getting the rainbow screen.
I’ve successfully installed and run raspbian using the same sd card and Pi, so I don’t believe it’s a hardware issue.
I notice the most recent image on the website and via the installer is 2018.02-1, should this be compatible with the B+ ?
No. If you can plug that card into another 3B (not +) and run the updater, it should then run fine in the B+. Otherwise, could be a day or two before the new image is available.