A few clarifying points:
I'm defining "work" here as any job where there is even a trace of a dress code. If you work part time at your campus rec center, I have a hunch you're already a seasoned sweatpants pro.
Sometimes you have to admit defeat. Know your audience. If you have to wear pantyhose to cover the ankle flash your pantsuit shows off, this probably isn't going to fly.
90% of this is a confidence issue. If you act like your sweatpants are office-appropriate, you're already most of the way there. Lounging is a only a mindset.
That being said, I strongly recommend either black or dark grey. In terms of fit, we're going for the opposite of leggings. These should not be sexual in any sense of the word. Try to avoid drawstrings at all costs.
Pants secured, throw on your most interview-friendly shirt and whatever footwear you feel classy in. Don't forget a belt. A half-tuck is probably also a good idea for extra booty modesty. Go to work. Act like everything is normal. Because it is - but better.