The passages on the left are in red text on green, both the red and the green in the first example being at the same YIQ brightness (see my text colour readability page for an explanation of this), therefore converted to monochrome both the text and the background should theoretically be the same shade of grey. Background is #008200, foreground #FF0000.

Though the should there may be significant, for the YIQ brightness values do not appear to relate to perceived brightness in the way one might expect, see my page Colour Workout. And possibly that is why the results we might expect do not seem to come true, but we’ll press on . . .

On the second example there is a YIQ brightness difference of -24. Background #008200 and foreground #AD0000.

I find that the effect of these close colour combinations varies quite a lot from monitor to monitor. On some screens, the first example is quite jarring and it’s hard to read the text, while on another the first is more easily legible than the second. This will presumably be something to do with the colour calibrations of the monitor, though it does raise a question around the legibility boundaries suggested by a readability formula (again see my text colour readability page).

Something else is happening too. In both samples there is on many monitors a yellow border or shadow to each letter, sometimes noticeably stonger on the example with the brighter red (the first) than on the darker one.

The passages on the right are in blue text on orange-red, the blue and the orange being at the same YIQ brightness in the top panel, and in the second panel there is a YIQ brightness difference of -24, as with the green on red samples above. The same considerations apply, the degree of readability, or perhaps we should say distinguishability, of the text varies from monitor to monitor.

On both of the blue-on-red samples, I perceive a kind of darker blue shadow on the south-west side of each letter. Presumably this is an optical illusion, that equates to the yellow shadow in the red-on-green examples.

With red on green, the yellow can be explained by additive colour mixing (presumably in the eye). You can see the effect of additively mixing colours on my Additive Colour Mixing page.

The orange-red is #C73100 and the blue is #0065FF in the first sample and #003CFA in the second. You’ll see if you try an additive mix that the result is a kind of pale lilac colour, so that doesn’t adequately explain why the shadowing should be dark blue. I have an idea this might be something to do with blue being a relatively non-bright colour in the scheme of things, though quite how this might be resolved mathematically has not yet come to me. Anyone any ’bright’ ideas?

And do these shadowing effects exist if you are someone with colourblindness? For if they do, then perhaps you can read the words, even though you theoretically shouldn’t be able to. Whether you have a colour perception deficiency or not, you probably don’t find it that easy to read the text, though with regular vision you probably can, just about, solely on hue difference, though whether this is solely on hue difference, or whether there’s actually an element of brightness difference there, is still a matter for experiment.

Of course, you wouldn’t normally use a text colour that was this close to the background colour if you wanted someone to be able to read something comfortably, but this is not the same thing as saying that red text on a green background is illegible, it isn’t, it all depends on the brightness difference between the shades of red and green, see my Red on Green is Readable page.

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