Coloring in coloring books and pages printed from ebooks has become a fad. It has been shown to be a relaxing practice and can produce some very beautiful works.
The idea is that there is no absolutely precise, pre-determined way to do coloring in coloring books. Whatever the person wants to do is going to be perfectly imperfect (to borrow a phrase from Monster High.) Coloring things however you want and use whatever you want.There are loads of crayons you can use (the Crayola crayons being the most familiar.)You can also use coloring pencils of which there are many choices and you can also use markers of a variety of types. You can mix what you use on any drawing to produce whatever coloring design you wish.
Amazon is a good place to find all sorts of coloring supplies and coloring books. I like the Dover publications and the Creative Design books especially but there are many others that you can get at very reasonable prices.
It's best to try to get ones printed on only one side of a page and the page used being of heavier paper but many books are done on both sides. In that case if you have a printer/scanner and you want to avoid bleed-over problems you can just scan the picture you want to color in and color that one, leaving your original book in pristine condition.
What I have below are drawings that I have colored in, taken from a variety of coloring books and separate drawings.
Alice in Wonderland
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