So I have been working on the Vector skin, and I have it change how I want, but I can't seem to find h1 to change the font color.
Topic on Skin talk:Vector
You want the class .mw-body h1 I believe.
Although just h1 should work fine?
Personally, I use #content {color: black !important;}, but that might cause unexpected problems.
Which file is this located in?
Oh, I thought you were referring to your personal stylesheet (e.g. Special:MyPage/common.css) or a browser extension that does CSS overrides (e.g. I use Stylus).
Are you working on a 3rd party wiki? Or trying to change the default for a single Wikimedia wiki? or for all Wikimedia wikis? If you have any relevant links (sites, discussions, examples, etc), they always help!
I am working on my own wiki. I downloaded the MediaWiki file and I have the site up, it just that I can edit everything else by going to the css file, but I did not see a .h1 section in the file in question.
Local overrides are usually just created in MediaWiki:Common.css E.g. m:MediaWiki:Common.css - that way you're not changing anything that needs to be auto-updated later (IIUC, i am not a dev).
For the origin, I believe they're in either https://github.com/wikimedia/mediawiki-skins-Vector/blob/master/components/common.less or in https://github.com/wikimedia/mediawiki/blob/master/resources/src/mediawiki.skinning/elements.css -- You might be able to find more with https://codesearch.wmflabs.org
HTH!