There are text shadows that are designed well, and there are text shadows are not so well-designed. And you can easily tell the difference when looking at text with poorly-designed text-shadows.
The simplest text shadows have only two parameters: one for the x-offset and one for the y-offset. The result often looks something like the line of text below.
This line of text is almost completely unreadable thanks to a poorly-designed text-shadow.
We could add a third parameter of color to see if that helps.
And we can clearly see here that adding a color to the text-shadow really did not help.
Perhaps adding a small blur-radius would help.
The blur-radius does make the text more readable, but this certainly is not ideal.
And that is because, even in real life, most shadows are dark in color. There are exceptions to this rule, but generally speaking, most shadows are dark in color.