Purple Climax on Black Box
#8800ff on #0f282f
Body text at this size
And the smaller size most interfaces actually use for paragraphs — the size that decides whether a pair is usable.
Caption text, which almost nothing passes.
WCAG 2.1
2.56:1
Fails AA
Not AAA (7:1). Swapping the two colours gives the same number — the ratio does not know which is the text.
APCA
Lc -21.4
Clears Lc 15 — disabled or purely decorative text, which is not meant to be read.
Light text on a darker ground — the negative sign is the polarity.
Colour difference
ΔE00 26.2
different colours
Above 10 — related only in the sense that both are colours.
What the text colour has to become
#a16dff is the nearest colour of the same hue that passes AA on #0f282f, at 4.50:1. Only the lightness moved, and some chroma was given up to stay inside sRGB — which is the point: your colour stays recognisably your colour.
And to someone missing a cone
protanopia
Ratio 2.11:1, ΔE00 30.8.
deuteranopia
Ratio 2.80:1, ΔE00 32.6.
tritanopia
Ratio 2.68:1, ΔE00 22.2.
The weakest of the three is tritanopia, where the two are ΔE00 22.2 apart against 26.2 for a trichromat — still clearly two colours.
Both colours have pages of their own: Purple Climax ·Black Box · the same pair the other way round ·all pairs