The safest bet is to conform with the World Wide Web Consortium’s (W3C) Web Content Accessibility Guidelines 2.0, which specify a brightness contrast ratio of 4.5:1 for regular text (12 pt or smaller), and 3.0:1 for large text. Contrast ratio is defined as:
[Y(b) + 0.05] / [Y(d) + 0.05]
Where Y(b) is the brightness (luminance) of the brighter color and Y(d) is the brightness of the darker color.
You calculate luminance Y by first converting each of the color’s RGB values to gamma adjusted normalize rgb values:
- r = (R/255)^2.2
- b = (B/255)^2.2
- g = (G/255)^2.2
Then combine them using sRGB constants (rounded to 4 places):
Y = 0.2126*r + 0.7151*g + 0.0721*b
This gives white a Y of 1 and black a Y of 0, so the maximum possible contrast is (1.05/ 0.05) = 21 (within rounding error).
Or let JuicyStudio do the math for you.
This calculation assumes a standard-performing monitor in a relatively dimly lit room (or a room that the user can make dim if she or he has to). That makes it adequate for home or office use, but I don’t know if it’s adequate for mobile apps or other devices that are used outdoors.