Generally the inks used in litho printing are transparent. In the case of process colour (full colour) printing, this is because the four ink colours used (cyan, magenta, yellow and black, known as CMYK) are designed to sit on top of each other to create all possible other colours. For example, 100% magenta and 100% yellow will create a strong red colour, but every printable colour will be made of some combination of percentages of these four colours, overlaying each other in a “screen” pattern of very small ‘halftone’ dots. These dots are too small to be seen with the naked eye, and combine in the eye of the viewer to create the effect of continuous tone colour. Process colour inks need to be printed one on top of the other in the correct sequence (KCMY) and at the correct ink strength (density) to accurately reproduce colours on press.
The fact that inks are transparent means that the viewer is actually seeing reflected light from the paper the inks are printed on – the highlight areas in pictures are very near to being plain white paper – which is why there is no white ink used in process colour printing (more on this in a minute).
Even pantone (spot) colours – which are usually intended to print on their own on items such as stationery – are generally transparent, and can be used in combination by an inventive designer (although it can be difficult to predict the results of making one pantone overprint a second one with any degree of accuracy).
However, there are a couple of exceptions to this… Metallic inks are opaque, which means they can be used to print onto coloured materials successfully. It also means great care has to be taken if using metallic colours in combination with other colours, especially CMYK, since, being opaque, they will either change the base on which the other colours sit (if printed first and left to dry) or cover up whatever has been created by the other colours (if printed last and overlaying the other colours). There is a related issue with printing halftones in metallic colours; which is that their metallic effect relies on the viewer perceiving light reflecting from the shiny surface of the ink, rather than the base colour of the paper below it. But if a halftone pattern is applied to a metallic ink, the viewer can see through the dots to the paper beneath, which can lead to some unexpected effects; as the sheet of paper is moved in front of the viewer, sometimes the eye sees reflective ink, sometimes paper and sometimes neither very clearly (when the sheet can appear suddenly and disturbingly dark).
Opaque white ink has been in use in screen printing and flexography for some years, and allows both simple use of white type on coloured or foiled backgrounds, and can be used in combination with other colours to create a whole raft of effects not possible with CMYK only printing. Recent developments in digital technology have also allowed the use of white opaque ink, which can, for example, be used to create a shaped base layer on transparent acrylic materials which is then overprinted with colour – giving the effect of an opaque image against a transparent background – something that was almost impossible to do in the past.