There is a color coding profile for JavaDoc doc comments where you can add this.
With a javascript file open, go to Document -> Javascript Options, and click on the "Color Coding" entry. At the top of the Color Coding page, there's a pull down for "Profile". Click on that, and select JavaDoc. Now it should show the @keywords for javadoc, and you can click on "Add Words..." to add @returns. And hit OK, and then @returns should be color coded in JavaDoc for all languages, not just TypeScript.