What kinds of things can be described as 丰富多彩 (richly colorful), and how do I describe them?
丰富多彩 is a combination of "丰富" (rich) and "多姿多彩" (colorful) 

丰富 is mainly used for describing something with rich substance or contents;  多姿多彩 is mainly used for describing something "colorful: entertaining; enjoyable"

Things that you can describe as 丰富多彩 (rich and colorful) including:

文笔，画功，剧情，生活，节目，故事 (Writing, drawing, plot, life, show, story) -- All these terms involve substance and enjoyment

文笔丰富多彩 (rich and colorful writing) - your writings is rich in substance and have a lot of flair 

画功丰富多彩 (rich and colorful drawing) - your drawings have a lot of contents and vibrant

剧情丰富多彩 (rich and colorful movies plot)- the plot of the movies is full of substance and entertaining

To be honest, I rather say 剧情丰富,多姿多彩 to make  the whole sentence idiomatic 

丰富 (rich) can be used literally (abundant) or figuratively (rich)  

多姿多彩 (colorful) is mostly used figuratively