I have used idye poly to make non-roll elastic a super intense vibrant yellow with just very hot water (boiled water in a kettle and poured into a container with an equal amount of hot tap water) but had difficulty dyeing to a rich black with lots of boiling and the correct amount.
I probably could have overdyed the deep green with more dye (I find for the really deep dark tones like navy or black require two to three times as much dye as is recommended regardless of the dye) but didn't.
I hope the attachment worked, I swear I am able to post photos on some browsers but not others...
So that yellow is actually even brighter. So jewel tones are a good and relatively easy level to get to really deep dark, not so much- more dye more effort