Text rendering with alpha in Color renders incorrectly

[brinko99]

There appears to be a bug with the rendering of text where the associated color contains an opacity (alpha). See the linked discussion.

@miloush noted that the issue is dependent on the font size and affects <Glyph> and <GlyphRunDrawing> as well.

Discussed in #7710

^{Originally posted by brinko99 April 11, 2023}
I’m trying to implement a text color in WPF that matches CSS RGBA of #00,00,00,0.72 on a white background. I’d expect that to be a color of #B8000000 (alpha of dec 184 ≈ .72), yet the result is much lighter than expected. Applying opacity to the control itself works as expected.

I'm expecting the composite color to be #474747 as produced by CSS and paint.net, but I'm getting #8C8C8C. Does WPF render alpha on a brush differently somehow?

Here is an example:

<StackPanel Background="White">
    <TextBlock Text="Medium Contrast (#B8000000)" Foreground="#B8000000" Style="{x:Null}" />
    <TextBlock Text="Medium Contrast (Opacity .72)" Foreground="#FF000000" Opacity=".72"  Style="{x:Null}"/>
</StackPanel>

[wstaelens]

This explains why XPS colors are sometimes diffent..