In practice, most printers make a trade-off, some opting for higher resolution and others settling for more levels per dot, the best solution depending on the printer's intended use. Graphic arts professionals, for example, are interested in maximizing the number of levels per dot to deliver 'photographic' image quality, while general business users will require reasonably high resolution so as to achieve good text quality as well as good image quality.
The simplest type of color printer is a binary device in which the cyan, magenta, yellow and black dots are either 'on' (printed) or 'off' (not printed), with no intermediate levels possible. If ink (or toner) dots can be mixed together to make intermediate colors, then a binary CMYK printer can only print eight 'solid' colors (cyan, magenta, yellow, red, green and blue, plus black and white). Clearly this isn't a big enough palette to deliver good color print quality, which is where halftoning comes in.
Halftoning algorithms divide a printer's native dot resolution into a grid of halftone cells and then turn on varying numbers of dots within these cells in order to mimic a variable dot size. By carefully combining cells containing different proportions of CMYK dots, a halftoning printer can 'fool' the human eye into seeing a palette of millions of colors rather than just a few.
In continuous tone printing there's an unlimited palette of solid colors. In practice, 'unlimited' means 16.7 million colors, which is more than the human eye can distinguish. To achieve this, the printer must be able to create and overlay 256 shades per dot per color, which obviously requires precise control over dot creation and placement. Continuous tone printing is largely the province of dye sublimation printers. However, all of the mainstream printing technologies can produce multiple shades (usually between 4 and 16) per dot, allowing them to deliver a richer palette of solid colors and smoother halftones. Such devices are referred to as 'contone' printers.
Recently, 'six-color' inkjet printers have appeared on the market, specifically targeted at delivering 'photographic-quality' output. These devices add two further inks - light cyan and light magenta - to make up for current inkjet technology's inability to create very small (and therefore light) dots. Six-color inkjets produce more subtle flesh tones and finer color graduations than standard CMYK devices, but are likely to become unnecessary in the future, when ink drop volumes are expected to shrink to around 2 to 4 picolitres. Smaller drop sizes will also reduce the amount of halftoning required, as a wider range of tiny drops can be combined to create a bigger palette of solid colors.