Hello,
As it seems there were quite a few discussion around highlighting meshes you can find below the solution integrated in standard in bjs:
var hl = new BABYLON.HighlightLayer("hg", scene);
hl.pushMesh(box, BABYLON.Color3.Green());
Please be aware, the stencil needs to be able in your canvas:
var engine = new BABYLON.Engine(canvas, true, { stencil: true });
You can take a look at the result here:
http://www.babylonjs-playground.com/#E3D3Y#2
Best Regards,