Calculate text color depending to a background color

Question

I'd like to create a variable for the text color, but depending to the background color that I've set.

:root {
  --main-color-hue: 205;
  --main-color-saturation: 73%;
  --main-color-luminosity: 29%;

  --main-color: hsla(var(--main-color-hue), var(--main-color-saturation), var(--main-color-luminosity), 1);
  --main-dark-color: hsla(var(--main-color-hue), var(--main-color-saturation), calc(var(--main-color-luminosity) * 0.5), 1);
  --main-light-color: hsla(var(--main-color-hue), var(--main-color-saturation), calc(var(--main-color-luminosity) * 1.5), 1);

  --main-text-color: red; /* calculate white or black */
  --main-dark-text-color: red; /* calculate white or black */
  --main-light-text-color: red; /* calculate white or black */
}

button {
  background-color: var(--main-color);
  color: var(--main-text-color);
  border: 0;
  padding: 16px;
}

button.dark {
  background-color: var(--main-dark-color);
  color: var(--main-dark-text-color);
  border: 0;
  padding: 16px;
}

button.light {
  background-color: var(--main-light-color);
  color: var(--main-light-text-color);
  border: 0;
  padding: 16px;
}

Main
<button>test</button>
Dark
<button class="dark">test</button>
Light
<button class="light">test</button>

Answer 1

You can consider the fact that a color with negative luminosity is always black and a white color is a color with luminosity bigger than 100%.

Here is an idea where I use calc(30% - luminosity) which will return a positive value if the luminosity is less than 30% (white color) and will return a negative value if the luminosity bigger than 30% (black color). I multiply everything by 100 to always have white in case of small positive value.

:root {
  --main-color-hue: 205;
  --main-color-saturation: 73%;
  --main-color-luminosity: 29%;

  --main-color:       hsla(var(--main-color-hue), var(--main-color-saturation), var(--main-color-luminosity), 1);
  --main-dark-color:  hsla(var(--main-color-hue), var(--main-color-saturation), calc(var(--main-color-luminosity) * 0.5), 1);
  --main-light-color: hsla(var(--main-color-hue), var(--main-color-saturation), calc(var(--main-color-luminosity) * 1.5), 1);

  --main-text-color:       hsl(0,100%, calc((30% - var(--main-color-luminosity))     *100)); 
  --main-dark-text-color:  hsl(0,100%, calc((30% - var(--main-color-luminosity)*0.5) *100)); 
  --main-light-text-color: hsl(0,100%, calc((30% - var(--main-color-luminosity)*1.5) *100)); 
}

button {
  background-color: var(--main-color);
  color: var(--main-text-color);
  border: 0;
  padding: 16px;
}

button.dark {
  background-color: var(--main-dark-color);
  color: var(--main-dark-text-color);
  border: 0;
  padding: 16px;
}

button.light {
  background-color: var(--main-light-color);
  color: var(--main-light-text-color);
  border: 0;
  padding: 16px;
}

Main
<button>test</button>
Dark
<button class="dark">test</button>
Light
<button class="light">test</button>

Another example where you change the threshold to 25% and notice how the first color will switch to black

:root {
  --main-color-hue: 205;
  --main-color-saturation: 73%;
  --main-color-luminosity: 29%;

  --main-color:       hsla(var(--main-color-hue), var(--main-color-saturation), var(--main-color-luminosity), 1);
  --main-dark-color:  hsla(var(--main-color-hue), var(--main-color-saturation), calc(var(--main-color-luminosity) * 0.5), 1);
  --main-light-color: hsla(var(--main-color-hue), var(--main-color-saturation), calc(var(--main-color-luminosity) * 1.5), 1);

  --main-text-color:       hsl(0,100%, calc((25% - var(--main-color-luminosity))     *100)); 
  --main-dark-text-color:  hsl(0,100%, calc((25% - var(--main-color-luminosity)*0.5) *100)); 
  --main-light-text-color: hsl(0,100%, calc((25% - var(--main-color-luminosity)*1.5) *100)); 
}

button {
  background-color: var(--main-color);
  color: var(--main-text-color);
  border: 0;
  padding: 16px;
}

button.dark {
  background-color: var(--main-dark-color);
  color: var(--main-dark-text-color);
  border: 0;
  padding: 16px;
}

button.light {
  background-color: var(--main-light-color);
  color: var(--main-light-text-color);
  border: 0;
  padding: 16px;
}

Main
<button>test</button>
Dark
<button class="dark">test</button>
Light
<button class="light">test</button>