Don't Use Colour To Indicate Binary States

Earlier this year I was playing around with DogeCoin. Try as hard as I might, I just couldn't get their API working.

I delved into the settings, to see what was up.

Take a look at this setting, is it obvious to you which state is active?

I clicked around on it, and the state changed.

Had this enabled it or disabled it?

By carefully hovering my mouse over the options, I could see what I thought was the active state.

Although, of course, I could have been wrong.

I don't know of any Human Interface Guidelines which specify that "Dark blue means on, light blue means off."

Here's the simplest way to do this in HTML

And, in code form

<form >
  <input type="radio" name="API" id="on" value="on">  <label for="on">API access is ON.</label><br>
  <input type="radio" name="API" id="off" value="off" checked="true">  <label for="off">API access is OFF</label>
</form>

Colour does not have any inherent semiotic value. Some cultures use white to indicate purity, in other cultures it's the colour of death. Pink, in my country, was a masculine colour until fairly recently.

If you're a designer or programmer, think carefully how your users will quickly identify the position of a binary switch.