Whether you are planning to buy a laser printer or an inkjet printer you need to think about the consumables you will need
Laser printers use toner cartridges, while inkjet printers use ink cartridges.
Toner for laser printers is essentially a very fine plastic powder. It is transferred to the paper using a process involving static electricity, and then heat is applied to fuse (melt) the plastic with the surface of the paper.
All laser printers use black (sometimes referred to as "K") toner, while colour laser printers also use cyan, magenta and yellow (CMY) toners.
Cyan, magenta and yellow can be combined in different proportions to create all the other colours. Well, not literally all colours. The pigments used to colour toner aren't perfect so any particular toner system allows a finite range of colours known as the gamut.
This approach to printing involves subtractive colour. That is, white light falls on the printed page and the toner(s) absorb certain colours. So when you see red, what has happened is that all the colours of the spectrum except red have been absorbed by the toner.
And that has two particular consequences.
Firstly, combining a full measure of cyan, magenta and yellow generally yields a rather muddy colour rather than pure black. That's one reason why colour laser printers use black toner as well as CMY toners. The other is that it is more economical to print black by using black toner than it is to print black by combining CMY. In most cases, a cyan, magenta or yellow toner cartridge is substantially more expensive than the corresponding black cartridge.
Secondly, white areas on the page are determined by the absence of toner. There is no combination of CMYK toners (other than 0 percent of each!) that reflect white light. That's not normally a problem, because we usually print on white paper. But what if you want to print white on coloured paper? Certain Oki models can use a special white toner cartridge allowing white print on coloured paper. This is particularly useful if you want to print chalkboard-style point-of-sale materials.
The Page Yield
An important consideration when selecting a mono laser printer or a colour laser printer is the number of pages you can expect to print with each cartridge. This is known as the page yield.
The reason page yield is important is that it is one of the main things that determines how much you will spend on printing. If you merely compare the price of replacement toner cartridges you may well come to the wrong conclusion. If one cartridge costs $200 and is good for 7,500 pages, that's a lower cost per page than a $100 cartridge that can only print 3,000 pages.
Another cost factor is whether or not a toner cartridge includes the corresponding drum (which is used to form the required image before it is transferred to the paper). A toner cartridge that incorporates a drum will almost always be more expensive than an equivalent one that doesn't. But once again, you need to look at the cost over time, because at some stage the drum will need replacing.
Imagine two printers that cost about the same and have similar page yields. One uses $120 toner cartridges with built-in drums, the other uses separate drums so the toner cartridges only cost $80. So after five cartridges the former seems to be $200 cheaper. But what if that's when the separate drum has to be replaced? If that costs more than $200, the combined toner/drum cartridges were cheaper after all.
But toner cartridges and drums aren't the only consumables involved in laser printers. When predicting running costs you may also need to factor in items such as transfer belts and waste toner bottles.
Much of the above discussion also applies to inkjet printers.
Like laser printers, they use CMYK printing. But there are two types of printer ink: pigment and dye. Dye inks are transparent, so you only get the expected results on white paper. Pigment inks are more opaque, so this is less of a problem.
That still leaves the challenge of printing white on coloured stock. There is such a thing as opaque white inkjet ink, but it is usually available only for specialist devices such as fabric printers and digital presses, not the inkjet printers generally found in homes and offices.
Another similarity between inkjets and lasers is that you need to consider the cost per page, not the cost per cartridge. A cartridge that costs $25 but only prints 180 pages works out more expensive than a $40 cartridge that's good for 400 pages. This comparison should be made between the printers on your shortlist, and between the different cartridges (standard, high yield, etc) available for your printer.
And just as some laser toner cartridges include the drum, the ink cartridges for some inkjet printers include the print head. Generally speaking, printers that use integrated cartridges are cheap to buy, but the cartridges are relatively expensive. It's not ironclad, but you'll usually find that printers with integrated cartridges only have a lower total cost per page for very low volumes. Otherwise you're better off paying more for a printer that uses separate cartridges or, for even higher volumes, one that uses high-capacity ink packs or even bottled ink.
Some people worry that buying a printer that uses a tri-colour (CMY) cartridge inevitably means wasting ink, as the cartridge is useless once any single ink has run out. But modern drivers for tri-colour printers monitor the use of each ink and subtly adjust the colour to even out consumption, so this shouldn't be a problem unless the documents you print are heavily skewed towards cyan, yellow or magenta.
Other inkjet consumables include printheads (if separate from the ink cartridges) and diapers (the pad that absorbs waste ink, especially during head cleaning). However, by the time these items need replacing, most people think it's time for a new printer.