Pro tip: adding black to yellow turns it kind of an olive green. If you want to darken yellow, try adding a bit of its complimentary color, purple. Something else to play around with while you're experimenting. Keep it up!

I tried adding black and it was just black with the smallest about of black. I did some reading online and found adding blue was listed, so trying that, that is what I used.
I'll try purple next time to see the difference.

Blacks are often considered dark blues (maybe only ivory black?).

You can also go the route of glazing!

Make layers and layers with transparent yellow. Just have to let it dry (really well with oil so one or two weeks, but it's godly when well done).