I couldn't find any references to this tool in the blogs or forums, so I wanted to put this out there.
I use A Web Whiteboard (AWW) found at https://awwapp.com for all my online tutoring needs. No download, registration, or install necessary... and it's completely free! It also has zero ads or any other clutter you might associate with any free tool. It appears the developers behind AWW make their money by selling a premium product to those interested in that sort of thing, but I've found the free tool more than enough for my needs.
It has pretty much everything I'm looking for in a whiteboard tool: multiple colors, incredibly simple to invite students to join the board, cross-platform (any student with an internet connection can use it, and it works in every browser, as far as I can tell), and there is an option to save the images you create so you and your students can have material for reference later.
The only conceivable drawback is ironically its simplicity. AWW's free version doesn't have drawing tools to help you draw straight lines, clean circles, and the rest (although it does have a text-insert option so you can type text in places where you don't want to draw out the words). I recommend taking some time to get comfortable with your mouse-drawing by yourself on AWW before introducing it to your lessons.