Floristry doesn't really have the best presence online, sure you can find plenty of pictures of bouquets and various other arrangements but it's surprisingly hard to find lessons, guides etc about it (special mention however to the MANY Youtube channels such as FlowerSchool or Flower Joos, those are great!)

You see, there's no "MDN of the flowers". Outside of the many videos on Youtube, typing "how to do [thing]" on your favorite search engine will most likely lead you towards sketchy websites or at least, really non-informative content

Thankfully, there's an alternative yet still true and tried way to learn anything that exist: Books

References

These books are not designed to be read from cover to cover, instead they should be used as references you consult when needed

Flower Color Guide

This book by Darroch and Michael Putnam is sometimes being referred to as being the "bible for florists". Case in point: When searching books about flowers, it will often come first or at least be heavily recommended!

The book advocate the idea of primarily identifying flowers by their colors and thus put a lot of emphasis on the theory of colors in floristry. It's a very elegant book, putting much attention on the flowers through full screen pages of them at their peak state

A followup book by the same authors focusing even more on the color theory also exist under the name "Flower Color Theory" (which I also recommend!)