In 2013, veteran video-game concept artist Paul Richards decided to put together an art book based on a fictitious IP of his own creation.  He invited several of his colleagues and friends to contribute to the book.  He also made an open call-for-art to the video-game art-world for contributions.  The only things required were skill and imagination.  There was no set lore or backstory to the IP... simply the premise of a story set in a dark-fantasy world.
Below is the process for an image I contributed to "Substrata"... Vox.
Original drawing vs. final digital painting
animated process... I struggled with this one a bit... it took a while to get a satisfying color scheme and textures... as you can see
final colors
You can find more about the book from Udon Publishing.  You can purchase the book from Amazon.
Disclaimer: Only the drawing made the book.  The colored version is something I completed after the call-for-art period concluded.