As part of "Documented Information", the ISO 9001 standard requires that you address "control of changes (e.g. version control)" and ensure appropriate "identification and description (e.g. a title, date, author, or reference number).
This means you have to have a way to identify which document is the right one, and which is the current version.
Usually, this means there is a document title (for identification) and revision information (for change control) on the document itself. You also have something like a document register that lists the current versions of all your controlled documents. This is to make sure that if I find a paper copy or keep a copy on my hard drive, I will be able to see what version it is and look up the register to see if it is the most up-to-date version.
For more details on what is required by the standard for document control, have a look at Control of Documents in our learning resources section.
Is it really necessary to use revision numbers AND dates?
No.
While 'date' is mentioned in the standard as example identification info, and there's nothing about revision numbers, it doesn't mean they're wrong. The standard does not specify what method you must use, simply that you have an effective method. You can choose what works for you. If you want to use colour codes instead of dates or numbers to mark revisions, that is completely up to you - but you MUST be able to demonstrate that it works effectively!
Keep your system as simple as possible!
For an example of a complicated scheme, here's the description of how version numbers increment in a scheme from "NIH.gov" (content is no longer available).
The first draft of a document will be Version 0.1. Subsequent drafts will have an increase of “0.1” in the version number, e.g., 0.2, 0.3, 0.4, …0.9, 0.10, 0.11. The first final version of a document will be Version 1.0. Include the date when the document becomes final. Subsequent final documents will have an increase of “1.0” in the version number (1.0, 2.0, etc.). Final documents undergoing revisions will be Version X.1 for the first version of the revisions. While the document is under review, subsequent draft versions will increase by “0.1”, e.g., 1.1, 1.2, 1.3, etc. When the revised document is deemed final, the version will increase by “1.0” over the version being revised, e.g., the draft 1.3 will become a final 2.0.
We recommend only using the date for your internal management system documents.
This makes it very easy to determine what the revision information should be, and your system documents don't change very often so you really don't need complicated revision number schemes.
Other documents like drawings for construction or manufacturing engineering drawings that do change more than once a day and will probably need revision numbers or 'Issue A, B, C.."
Note that even if you only use dates for your internal documents, your Document Register will still need a column for both numbers and dates to allow you to track external documents with revision numbers.
Quality Systems Toolbox has a place for both revision date and number, but you can choose whether to use either or both. It doesn't track every single edit you make to a 'draft', so they have no numbering. When you approve and release a document, the revision details are shown on the register and follow that version as it is superseded by a new release.