colourful sidebar?

Yes, but there aren't any that the Finder will support without code modification. Given the security measures implemented in macOS such mods are not possible. There used to be a third-party utility that did implement colored icons but I think it is no longer supported.

Hi Benji, while tinkering with the OS isn't officially encouraged by Apple there are ways. Especially for advanced users who like to customise their system.

You would need to temporarily disable System Integrity Protection (SIP) to activate a change like you want. Please understand that SIP is an important part of keeping your system secure and shouldn't be left disabled or tinkered with if you're not sure what you're doing.

Having said that, you can try out tools like XtraFinder or/and MacForge.