In the sitatuation where the user can select any Color as BackgroundColor:
What kind of algorithm/detection do you suggest to detect which FontColor (black or white) is "best readable" on that (userdefined) BackgroundColor?
If you even have a code snipplet: thanks even more ;)

Dim dVal As Double = (mColor.Red * 0.299) + (mColor.Green * 0.587) + (mColor.Blue * 0.114)
  if (dVal <= 127.5) then
    'Dark Background: Bright TextColor
    colBestForText = &cFFFFFF
  else
    'Bright Background: Dark TextColor
    colBestForText = &c000000
  end if

An attempt with "perceived Luminance" (according to: https://www.w3.org/TR/AERT/#color-contrast)

Color brightness is determined by the following formula:

 Dim dVal As Double = (mColor.Red * 0.299) + (mColor.Green * 0.587) + (mColor.Blue * 0.114)
  if (dVal <= 127.5) then
    'Dark Background: Bright TextColor
    colBestForText = &cFFFFFF
  else
    'Bright Background: Dark TextColor
    colBestForText = &c000000
  end if

Is this good enough - or what do you use?

Unless you are going to color your background (not using black or white), I would stick to the guidelines of macOS and windows. The link you provide is a draft document of the year 2000, and has not been updated to current standards as far as I can judge. Let the system handle it, i.e., light and dark mode on macOS and high-contrast on windows. Apple and Microsoft do know about contrast guidelines.

Dave, it is all about color vision and some suffer from not being able to see colors. Yes I understand the thread, there's no need to start a discussion about whether or not I would understand it.

Color blindness is an issue and requires further considerations, suggesting, for example, to refrain from using a colored background. That's all.

@Jürg O An attempt with "perceived Luminance" (according to: https://www.w3.org/TR/AERT/#color-contrast )

Looks good to me. Only thing I would change is to move the threshold up to 180 or so. I feel like it's easier on the eyes to look at white on medium/light gray than black on gray.

@jim m Looks good to me. Only thing I would change is to move the threshold up to 180 or so. I feel like it's easier on the eyes to look at white on medium/light gray than black on gray.

Thanks for this input!

@Dennis H let them select the font color.

@Dave S there may also be situations (not specificly related to the OP) where the background color shifts based on program logic, and the text color needs to compensate as well

Quite close :) It has nothing to do with DarkMode. It's for the situation where the Background in some "table cells" get different colors (a combination of user-selected and some "business-logic").
And there is "Text" that needs to be drawn on top of that BackgroundColor. Not red/blue/user-selected - just "plain Text".

So it seems to me that the possible solution mentioned above works good enough for this situation.
And I appreciate the Input about why you suggest to use a different treshold than "50%".

@Jürg O An attempt with "perceived Luminance" (according to: https://www.w3.org/TR/AERT/#color-contrast )

 Dim dVal As Double = (mColor.Red * 0.299) + (mColor.Green * 0.587) + (mColor.Blue * 0.114)
  if (dVal <= 127.5) then
    'Dark Background: Bright TextColor
    colBestForText = &cFFFFFF
  else
    'Bright Background: Dark TextColor
    colBestForText = &c000000
  end if

Is this good enough - or what do you use?

thanks for the code above, Jürg Otter.

i have put this code into my application where user are allow to change the background color or the sidebar and check it is suitable for black or white.

i make it into a function and call it from the bevelbutton that open the color selector

Public Function TextColorBlackorWhite(mcolor as Color) as Color
  Dim dVal As Double = (mColor.Red * 0.299) + (mColor.Green * 0.587) + (mColor.Blue * 0.114)
  If (dVal <= 127.5) Then
    'Dark Background: Bright TextColor
    Return &cFFFFFF
  Else
    'Bright Background: Dark TextColor
    Return  &c000000
  End If
End Function

If TextColorBlackorWhite(c) = kkWhite Then
      MsgBox "You need a color that goes well with black color text"
Else
      ....
END IF

@Dave S The only thing "darkmode" will do is swap BLACK for WHITE if those are one of the choices you assigned.

Not according to Apples HIG: https://developer.apple.com/design/human-interface-guidelines/macos/visual-design/dark-mode/ ;)

@Jürg O Only thing I would change is to move the threshold up to 180 or so.

where to change the value to 180??? is it the 127.5?

@Richard D where to change the value to 180??? is it the 127.5?

Yup. dVal will be between 0 (dark) and 255 (bright). Use any value in between to switch - my example just uses the "middle" (127.5). If you'd like to respect Jim's Feedback, change it to a higher value.