[pdf embed api] highlight annotation default color

Yes. Look at the "startAnnotationMode API" section of the documentation. The code will look like this except you'll be generating your own color.

const mode = "highligh";
const options = {
  defaultColor: "#b80000"
};

previewFilePromise.then(adobeViewer => {
        adobeViewer.getAnnotationManager().then(annotationManager => {
                annotationManager.startAnnotationMode(mode, options)
                        .then(result => console.log(result))
                        .catch(error => console.log(error));
   });
});