Product: PDF WebViewer
Product Version: v11.1.0
In the docs, I easily found how to modify the annotation color.
const { documentViewer, Annotations } = instance.Core;
const highlightCreateTool = documentViewer.getTool('AnnotationCreateTextHighlight');
highlightCreateTool.defaults.StrokeColor = new Annotations.Color(236, 0, 0, .5);
But I didn’t find any reference about the default blue highlight when selecting text.