The site I use often with my students is ArtyFactory located at https://www.artyfactory.com/. I experimented with a number of the visual impairments that were included with the filter and discovered that in a number of instances the site managed a lot of the more minor visual impairments with reasonable efficiency. The more severe impairments however really impact the usage of the site. They could benefit from including some additional accessibility options.
In other aspects the site does allow for easy reading and easy following of information and of direction for the lessons. Though to be fair a visual art learning site should try to think of those who may want to try learning how to paint for sighted audiences.