Mustard is very popular. I've done that with good results.
I put a new handle on an old knife for a friend. It was a large knife - almost a machete - and they used it exclusively for cutting watermelons. They had used it for at least 50 years. It had the most wonderful patina I've ever seen. Almost like an OD Green, like old military stuff.
On my carbon blade knives, I wet sand with 800 grit, then wash off, dry and apply mustard. I rub the mustard in before coating and letting it sit for 4 hours. Rubbing it in helps fill some of the rougher areas of the blade and reduces patina streaks. After the 4 hours, I wash off and dry.