The standard flow of a book project goes as follows:
Step 1: Read over the illustration details, and review preferred art style and reference images provided.
Step 2: URGENT Provide a draft sketch of ONE of the illustrations in the story ASAP (it must include the main character)
Step 3: Once the draft is reviewed and approved, you would color that page and send for a color review
Step 4: Once you provide the first page in color you can also immediately start working on the remaining sketches for the book. These you would complete all at once and provide as soon as you can for review.
Once they are reviewed and you complete any edits, they can then all be colored at once.
They then get a final review and approval.