I am a huge supporter of affirmative action, and I'm glad you're trying to do it, but you're doing it the wrong way.
Who follows you on Twitter? Do you have a big black/brown following there, or is it mostly white people? If it's mostly white people then you should just accept that's a white marketing channel, and don't go trying to find black folks there.
Now ask yourself: what parts of the Code For America community are substantially black? Do you have teams that have better representation? Do you do events that have better representation? You can reach out there, without even mentioning race, and get a higher probability of black applicants. Just send an email "We're hiring programmers of all experience levels! Please forward this to friends/post on Facebook/etc!" You'll get some good white applicants in there too! Great! Hire them! But hopefully it's a better racial mix than whatever you're getting with your existing marketing channels.
If you're a pretty white organization, you might have to look further. Look at your city. Where do black folks congregate? Where do latin american folks congregate? Go to those places, and again, just do regular marketing. Not "Any brown folks want to work for us on prison stuff?" but "Anyone want to work with us on the wide array of activities we do?"
Maybe they don't even care about prison reform. Frankly, that is a little racist. Lots of black people don't. Maybe they like your mission, they care about voter turnout or something, but they're happy to apply for a job in the accounting department.
Now you've got 1 more person of color on your team, that will make all of your future job marketing activities work better.
Please be careful here, affirmative action is not a straightforward concept, and it's easy to alienate both white people AND people of color. There's a right way to do it and many very wrong ways to do it.
Feel free to email me if you need someone to bounce ideas off of.