Personally, I'm not a big fan for white undercoats for yellow, as your undercoat will allready have to be very smooth and equal to get a nice, clean yellow.
Personally, I found the best results were either starting from an Orange (Fiery Orange for example) or one of the more ochre browns, like the Bubonic Brown, Iayden darksun or Tausept Ochre.
Oranges are good for a vibrant, lively yellow, whilst the browns might be a better solution for Orkses.
Now the important bit is to make sure every layer is nice, equal and smooth, no streaks or darke/lighter patches, as these will shine through up to your actual yellow finish.
After the base-colour, I mix up approx 3 measures of the yellow I want with 1 measure of the base-colour for a first highlight, and then a pure yellow for a second highlight.
then, finishing touch, take 3 measures of Skull White for 1 measure of yellow, and highlight the extreme edges with that.
A nice wash to make the yellow more alive and warm is Gryphonne Sepia.
On the armour of these Orks, you can see what the approach starting from Tausept Ochre looks like :