ONE BY ONE, monochrome corners of office life are being brightened by electronic splashes of colour.

Computer terminals went first, almost 15 years ago. Personal computers followed in their turn. On paper (and acetate), plotters led the way, succeeded by different types of computer printer. Full colour photocopying is not likely to expel mono from many offices in the near future, but it is next in line.

The justification for colour lies in the impact it has on its recipients. According to research, it is 32 per cent more effective in retaining the attention, and 22 per cent more effective in generating a positive response.

In the office, the colour versions of earlier monochrome machines were at first regarded as items of expense requiring careful justification. In most cases, prices have fallen as volumes have risen, and colour is now more likely to be taken for granted.

Colour commands a premium, but the tendency is for the premium to shrink. On monitors for personal computers, it has effectively disappeared. Eventually, perhaps, black and white will be more expensive, as it is in 35mm film processing. In the less competitive world of Apple computers, the colour premium on monitors is 80 per cent or more.

With output machines such as printers and photocopiers, the extra cost of colour comes in two installments: the difference in capital cost, and in the cost per page. The difference in price between mono and colour printers is difficult to express because direct equivalents are not easy to find. As a sample, Kodak's Diconix 330C colour ink-jet printer costs pounds 799, against the mono Diconix 180si at pounds 299. But the comparison should perhaps be against alternative means of producing colour output. Kodak's Nick Richardson, manager, electronic products, points out: 'We have a top-of-the- range printer whose output costs pounds 2 a page, and with mono you might be talking about a few pence.'

But customers see colour presentations as a possible key to closing major deals, he says, in which case pounds 2 is a modest outlay. Besides, a colour photocopier produces output at 25p a page, as opposed to the pounds 30 that a chromalin might cost.

In photocopying, Kodak's business development manager, Ross Dorras, believes that while full colour costs anything up to 25 times as much per page as black and white, a technological breakthrough will be necessary before colour makes a real impact. The same could be said of colour screens on portable computers. But in each case there are promising halfway houses, in 'highlight' colour on copiers and low-cost production techniques on PCs.