So this is my first time posting something to this booru and I'm not sure what to tag as the source. For one thing it is technically an edit of something already on here, but it's an edit that I made and have not posted anywhere else. Do I tag the original as the source or do I mark it as no available source? Sounds like a dumb question, but the FAQs and help section don't have anything on it.

what you're supposed to do for third-party edits is fill the box that says "parent" with the post number of the original that you've edited, if the og isn't on here already than you might want to upload the original first and make sure there's not a good reason why it wasn't in the first place

as for sources: there aren't any hard and fast rules for sourcing an edit. i'd recommend adding your edit as a source if it's available on other sites, but i've seen other edits just lead back to the base art or even just nothing at all. again, the parent/child relationship is much more important

There are tags that you should use when uploading that stuff, although take this on advisement because this assumes you have the following:

a) You have permission from the original artist who made the content you're editing
b) It is acceptable content to post on this website

The tags you're likely looking for are color_edit and third_party_edit. Color edit implicates third party but if it's not a recolouring you should just use third party edit.