Creating your own BBCode?

On some forum sites it's given me descriptions of how to make my own, for example:

[rainbow]This text should be in rainbow font[/rainbow]

Anyone know how? Thanks <3

Sorry, not possible.

Well I've seen people with it, so it has to be.

I think this is the closest you can get for now.
https://help.forumotion.com/tricks-tips-f32/add-more-bb-colors-to-your-forum-template-editing-t52539.htm

Hello,

it is semi-possible with serious CSS3 & Template coding combined to do so.
So only possible on PunBB & PHPBB2.

I love jQuery so yes I am bias but I think this is something that can be done with jQuery, though it would take quite a bit of effort. It's something I'm exploring myself actually, if I could figure out exactly what prevents html from working in my posts even with html turned on or what prevents my attempts at using jquery to style things within a post.

But I have had luck finding and replacing things within a post so once I figure out more of what's working how I will see if I can make up a quick bbcode plugin.