For pretty nearly worst-case performance, try reading text on your computer screen written in either pure bright red on blue, or vice versa. The problem is that your eye is not achromatic; it focuses different wavelengths differently. On a notionally flat background, like a computer screen, any such difference in focus stands out abruptly and rapidly becomes very wearying and irritating. Looking for red berries among green leaves is a totally different matter, because in nature there practically always will be differences in perspective that overwhelm subtleties in focus. If you experiment on the computer screen, you will find that the worst effects result from large differences in wavelength between pure colours; mixed colours, especially when composed of little dots as in pointilism, and when their differences in wavelength are small, don't cause any obvious difficulty at all.

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To add on to what Jon has written...

It is clear to everybody that black text on a white background is extremely easy to see, which is part of the reason why most of the printed material you read is black text on a white background. Likewise, white on black produces high contrast, but it is more difficult to read because black is perceived as being heavier than white and, thus, squeezes it out a little.

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It is possible to use coloured print on a coloured background, but it's not usually successful outside of the realms of art unless you make the two colours of differing saturation with the lettering the most saturated & the background 'washed out'.

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Bad ideas relating to optical effects & colour blindness:

* Red and green used together where you need to distinguish between the two colours to understand the content -- red / green colour blindness is the most common

* Blue and yellow used together where you need to distinguish between the two colours to understand the content -- 1-2% of men have blue / yellow colour blindness

* Using different blues simultaneously, fine blue text or slender blue line diagrams -- blue sensitive cones in the eye are the least numerous (blue is fine for large areas)

* Pairing extremes of wavelength (opposite ends of the colour chart) together e.g. red and blue -- headache inducing combinations

* High chroma colours together -- spectrally extreme colours, e.g. blue and yellow, can cause perceived vibration, imaginary shadows and more headaches

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Michael

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