Thanks to Sharon Tingey who shared her secret mix of colours in her demonstration and her Facebook post, I found a solution to my frustration of painting yellow flowers. And I hope she doesn't mind I write here how her great tips help me.
I know from experience that yellow flower is not as easy as red, pink, purple, or orange flowers to deal with. I used to stick with W&N Cadmium Lemon or Cadmium Yellow Pale for yellow flowers, whose result did not make me happy. I felt that the more layers I put to built shapes and shades, the less bright the yellow became. It was very tricky for me. A very tiny amount of wrong pigment in the mix would turn the whole flower dull. And the mistake was hard to hide.
But the misery ended after I watched Sharon's demonstration and saw how well she did her sunflowers (which won Gold Medals in RHS London Botanical Art Show 2014!). I bought and tried the same paint she used. And I felt very grateful that she shared her recipe! It was Daniel Smith Hansa Yellow Light (HYL). As you can see in the photograph above, which compares yellow colour chart, HYL is the brightest among other yellow colours I use. The DS site claims that the paint is cleaner, more transparent and brighter in chroma than Cadmium Yellow Light. Also I guess because HYL uses a single primary pigment, it helps us avoid a dull gray resulting from hidden complements when mixed.
A mix of HYL with W&N New Gamboge or Quinacridone Gold or and Perylene Maroon (for the darkest) really captures the brilliant yellow petals of sunflower.
And below is a scanned image of my Sunflower illustration. I hope you like it! :)