This functionality is available on Onshape's browser, iOS, and Android platforms.

Using the context menu for a specific part (or group of selected parts) you can customize not only the color of the part, but also assign a material (and thereby, a density), as well as specify a custom density without assigning a material. Onshape provides a material library for your use, and you are also able to add (or remove) your own custom material libraries.