Bills can have three statuses - draft, complete, or void. The below tell you what each one is
Draft - pink - this is a bill that has been started but not completed. It is saved in your incomplete bills, against the booking, and on the client's profile
Complete - green - this is a bill which has been completed and finished processing. These bills make up your reports
Void - grey - this is a bill that has been voided and thus removed from your reports. You can reprocess these with corrections by clicking 'Copy to new bill' on this bill