Because the font has no color. A control can display text using font and color, but color is not a font property.
EDIT:
If you want to use a text box that uses the given font and color, you can do the following (I assume you are using winforms):
var myTextBox = new TextBox();
myTextBox.ForeColor = col;
myTextBox.Font = birthdayFont;
myTextBox.Text = "Happy birthday!";
this.Controls.Add(myTextBox);
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