https://magma.com provides a web interface for drawing and editing images and it seems to be collaborative, i.e. you can share a link with someone to work together on an image!
However, it can be kind of painful to do conventional 'drawing' by dragging a mouse. I think it might be more enjoyable to do a paint-by-numbers thing where you fill in white areas by clicking with a paint bucket tool. You can find a huge variety of black/white line drawings at https://www.justcolor.net/.
So we have the images and the colouring platform. What could go wrong? Well, most online images, including those in the link above, tend to have grey pixels between the black and white bits. This is manifested as the paint bucket 'not filling in the whitespace entirely', like this: