Practical
What to colour a printed page with
A sheet from your own printer is not the thick paper of a bought book, and ink knows it. This is what works on the pages here, in the order the problems arrive.
Most supply guides are written by people who colour in books. A page you printed at home is a different object: thinner, whiter, and lying loose on a table rather than bound into a block of paper that soaks up whatever gets past the top sheet. That one difference decides almost everything below.
Start with the paper, not the pens
The paper in your printer is almost certainly 80 gsm, sold as 20 lb in the United States. It is fine. Crayons, coloured pencils and children’s felt tips all sit on top of it and stay there. But every marker made for adults is formulated for heavier stock, and on 80 gsm the ink goes through the sheet, through whatever is under it, and into the table.
One ream of heavier paper solves it permanently, and it costs less than a set of pens. Look for 32 lb, which is 120 gsm. Anything at or above that stops markers. Card stock at 160 gsm is worth it only if the finished page is going to be cut out or taped to a wall.

Hammermill Premium Color, 32 lbPaper
Thirty-two pounds is 120 gsm. This is the sheet that stops a marker reaching the table underneath. Ordinary copy paper is 80.
Markers: alcohol or water-based
This is the question the whole hobby argues about, and it has its own page, because the answer decides the paper as much as the pens. In one line: alcohol markers look better and go through the sheet, water-based ones stay on it and buckle it slightly. Which one suits you, which sets are worth the money and what a brush tip changes are all on the markers page.
Coloured pencils, the forgiving option
Pencils are the only medium on this page that works on paper you already own, and the only one where a wrong colour is not permanent. They are also slow, which is the point for anyone colouring to wind down rather than to finish.
The difference between a school pencil and an artist pencil is wax. A soft, wax-heavy core lays down thick enough that a second colour on top mixes with the first instead of sitting beside it, which is what people mean by blending. That softness is also why the leads wear down and break faster. If nobody is blending, the expensive set is a waste of money, and the fifty-colour box is genuinely the better buy.
Between the two sits the sort of set that costs about a pound a pencil: wax-based, soft, seventy-odd colours in a tin. Which one, and when the cheap box is the better buy, is on the pencils page.

Arteza Pro Series, 72 coloursPencils
Wax cores soft enough that a second colour mixes into the first instead of sitting beside it, which is the whole difference with a school pencil. Seventy-two of them, in three trays.

Crayola coloured pencils, 50 coloursPencils
Fifty colours is past the point where a child asks for one that is not in the box, and the leads survive being dropped on a tiled floor.
The printer, if you are printing a lot
A coloring page is pure black line on white. That is the one job a cheap monochrome laser does better than any inkjet: sharper edges, no bleed into the fibre, and a cost per page low enough that nobody has to ration the drawings. Toner also does not dry out, so a printer used twice a month still works on the third month, which is not true of an inkjet.
If you already own an inkjet, keep it and read the printing guide instead. Most bad prints are settings, not hardware.

Brother HL-L2400D, mono laserPrinter
A coloring page is pure black line, which is what a laser does well and an inkjet does expensively. Toner also survives three months of not printing.
When you would rather buy a book
Everything on this site is free to print, and there are still evenings when nobody wants to switch a printer on. A book also comes on paper thick enough for markers, which quietly removes the whole first section of this page. Which ones are worth it, and the one specification to check on the back cover, are on the coloring books page.

Disney Mystery Coloring: The ClassicsBook
Colour by number, and the character only appears as you go. It holds a child who gives up on an open page, because there is something to find out.

Secret Garden, Johanna BasfordBook
The book that started the adult coloring shelf, and still one of the most detailed. Printed on stock heavy enough for markers, which is half of why people buy it.
What to buy first
If you are starting from nothing and colouring with children: the fifty-colour pencils and a box of washable markers. Nothing else. If you are colouring for yourself and want the finished page to look like the ones you have seen online: the heavier paper first, then the markers. Buying the markers before the paper is the mistake almost everybody makes.
Then go and print something. The category index has the whole catalogue, and every sheet is a 300 dpi PDF made for A4 and US Letter.
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Supplies questions
What is the best thing to color a printed coloring page with?
Coloured pencils, if you only own one thing. They work on the 80 gsm paper already in the printer, they never bleed through, and a mistake can be softened instead of thrown away. Markers give a better finished page, but only once the paper underneath is heavy enough for them.
Why does my marker go through the paper?
Because ordinary copy paper is 80 gsm, and every marker beyond a child’s felt tip is designed for heavier stock. Print onto 120 gsm and the problem disappears. If you would rather not buy paper, slide a spare sheet underneath and accept that the page is one-sided.
Do I need expensive pencils?
No, and for children the cheap ones are the right answer. The difference between a school pencil and an artist pencil is the amount of wax in the core, which is what lets two colours blend into a third. If nobody in the house is blending, that money buys nothing.