According to the blog Modern Grace Kelly, Laverne Basham did Gillian Anderson's makeup during her years on "The X-Files." And it seems that Basham's go to lipsticks were MAC's SHHH, Philosophy lip stains, and the Chanel Aqua Crayon Lip Liner in the color Cognac, according to TikToker Brittany Cale (via TikTok). As Cale points out, these have long been discontinued (sadly), but it's actually not as difficult to find alternatives as you may think.
What Cale seems to lean towards is shades in the peachy-pink hue, like MAC's Satin lipstick in the color Retro or Spice lip liner which was used in a lot of movies in the '90s including "Scream" and "American Beauty." There are plenty of other options if you're not fully sold on the MAC idea, though. Modern Grace Kelly suggests Rimmel's Kate Moss Nudes in #47, Revlon's Toast of New York, and Maybelline's Maple Kiss.
Essentially, when it comes to Scully's lip shade it's all about your neutral pinks and reds, with a little peach thrown in and even orange (via The Lipstick Duchess) for good measure.