Most of the experienced users on this site don't use NOOBS - even on the Foundation Forum experienced users tell new users not to use it - I don't know why the Foundation keeps recommending it.
Unless you have something worth recovering I suggest you use Etcher to install Raspbian to a SD Card (like the vast bulk of us).
If you get to a recovery console ...
Pi4 Boot Problems Sticky contains detailed troubleshooting steps.
Using NOOBS just makes fault finding harder.
The rainbow screen is due to missing (or obsolete) firmware.
I suggest you use Etcher to install Raspbian to a SD Card (like the vast bulk of us).
You can do it by mounting the NOOBS partition, usually is the first partition in disk, and adding the following entries, in this partition, at the begining of the first line of recovery.cmdline file runinstaller and at the end vncinstall.
runinstaller is for booting into NOOBS installer.
vncinstall is for enabling vnc on NOOBS.
runinstaller quiet ...
This is the wrong site to ask about NOOBS because few (if any) of the experienced uses use NOOBS. The general recommendation on this site is NOT to use NOOBS.
You should be able to re-install Raspbian from NOOBS by pressing the Shift key at the correct time during boot.
Did you update Raspbian BEFORE upgrade? The following command will update Raspbian.
NOOBS has a recovery option to trash everything you've done on your Raspbian system and start again (it gives you an unpatched system just like the day you started).
Hit or tap the SHIFT key during boot to activate the recovery menu.