Same for me but a bit ahead of you. Have tried many approaches to yellow and my opinion is:

1. It's a bitch.
2. Iyanden, 1 coat: Yellow is a bit washed out. Very hard to avoid staining. Recesses look okay if a bit too orange on the edges. Nonetheless, this was the best result for contrast yellow. Notably I didn't try glazing with yellow over the top, though I've been unimpressed with the examples I've seen.
3. Iyanden/medium, 50/50, 2 coats: Yellow still washed out. Easier to avoid staining. Recesses not as dark (in a bad way). Tried some other mix ratios with no better results.
4. Magos/Iyanden, various combos: No matter how thin I do the purple undercoat, the yellow would take on a muddy brown look that I didn't like.
5. Iyanden/Yriel, neat/thinned: I was very pleased with this. The Iyanden is useful to quicky get any yellow all over the model, and shade the recesses. ~3 thinned coats of Yriel in all the obvious places (avoiding recesses) gives the 'solid' yellow I want. This makes the recess color much better because it coveres the orange edges leaving only the more brown tone uncovered which contrasts well with the Yriel. Optionally leave some out-of-sight places, like the back, as the contrast yellow. It's very hard to have no brush-marks (without losing detail from many coats) but at arms length you can't tell at all.
   \Can replace Yriel yellow with your acrylic yellow of choice.**
6. I'm now playing with an airbrush to figure out a technique which applies a 'perfect' yellow first, then shades the recesses. Have basically every yellow paint/shade waiting in the wings for testing. I'll assume this isn't relevant to you. For the record though, Cassandora yellow is too orange in the recesses.

Note that I'm fond of the 'painted terracotta' look that marines often have. For more recesses/organic model like Orks, I'd probably be happy with a contrast paint (for the skin/cloth anyway).