Measuring Contrast Instead of Guessing

The design reference for this theme carried contrast ratios for every accent colour, worked out by hand from the sRGB values. When they were finally measured against the surfaces they actually sit on, several were wrong — and the ones that were wrong were the ones nobody would have questioned.

What failed#

Comments in code blocks measured 3.28:1 in light mode and 3.85:1 in dark, against a floor of 4.5:1. Light-mode numbers measured between 4.08 and 4.32:1 depending on the palette.

comment  light  #8a8a92  3.28:1  ->  #727278  4.60:1
comment  dark   #71717d  3.85:1  ->  #7e7e89  4.60:1
number   light  #b06a1f  4.08:1  ->  #a4631d  4.60:1

Why hand calculation goes wrong#

Relative luminance is not lightness, and the green channel carries 72% of it. Two colours that look equally dark can differ by a factor of two once weighted, which is exactly the error that puts a grey at 3.3:1 while it reads as comfortably mid-tone.

Measure against the real surface#

Code sits on --surface, not on the page background. A comment grey measured against white clears the floor; the same grey measured against the code block it actually sits in does not. Sample the rendered page, not the palette.