Coloring books are a huge market, with repeat buyers.
And you can sell your own much easier than you'd think.
With Amazon KDP self-publishing, you just have to fill out some basic information like the title & description, and make a file for the book interior & cover.
When copies sell, Amazon prints the books and delivers them with Prime shipping. You collect royalties every month.
Depending on page count, you can price them at $4.99 (free Prime shipping included) and make $0.84 profit each.
I use Canva to design my covers & interiors. They have a paid Pro plan with great extra features, but you don't need it for KDP designs. There aren't a ton of graphics for coloring on the free plan though.
For more graphics, I highly recommend Creative Fabrica. While a member, you can download as many of their files as you want--and there's always more being uploaded. At least try the free 1 month trial.
At the moment if you use my affiliate link, you can get access for 1 year for $79.99. That's less than $7 a month. It's a great value--especially if you also use their graphics and fonts to sell print on demand t-shirts on places like Amazon Merch, Redbubble, and Etsy.
The hardest part (to me, anyway) is making a good looking book cover design. But you can pay someone on Fiverr to design one cheaply, or join Vexels and use one of theirs.
Plus, you can always update the cover and interior if the book isn't performing well or you come up with a better design.
The only thing you can't change once you publish is the title&subtitle. So you'll want to do some keyword research to find a good SEO-oriented title. A good idea is to niche-down to get very specific--eg 'Easy Halloween monster coloring book for kids ages 4-8'.
Niche-ing down means you'll have much less competition so it'll be easier to show up at the top of search results on page 1, and make sales.
For a guide with all the details--and no fluff--I have an affiliate link to a great ebook, Coloring Book Masterclass by That KDP Guy. It's how I learned to do it.
It's at steal at $19.99. You should make that money back pretty quickly--especially if you also create other books like puzzle books, joke books, etc.
You can even create basically blank books, for people to write/sketch in. I also have an affiliate link for a great video course that shows you how to do every step of the design and upload process, starting with blank books--it's called Print That Profits, and comes with some bonuses and ready-to-publish 'done for you' interior files.
But since blank books are so easy, there's a lot of competition--you'd have to upload a ton to make decent money. I'd focus on coloring books, since they seem hard to make, but are pretty easy. And since they're such massive sellers--popular among toddlers, kids, teens, and adults. (A big niche for adults is curse/swear words on top of mandala patterns.)