Bought this one myself, but it's not official. That said, they've simply taken art images and turned them into coloring pages. Badly, at that. However, because of that, I decided not to level the images as it damaged the picture quality overall. Now, get coloring.
Direct Download - https://mega.nz/#!lcdH3SKa!jHH0nKEg9xroJmIPib32m0Z1pxLLI6O7-imFprvkCPw
[xPearse] Honkai Impact 3rd Colouring Book [RAW SCAN].zip (159.4 MiB)