textBox1.ForeColor = Color.FromArgb(255, 0, 0); // color rgb selector here https://www.google.com/search?client=firefox-b-d&q=rgb+color+selector
or
textBox1.ForeColor = Color.Red
or
textBox1.ForeColor = Color.FromKnownColor(KnownColor.Red)
  or
textBox1.ForeColor = Color.FromName("Red")