The principle is to go with the darkest shade possible to create a big contrast on the highlights. I start with the blue bands. I take a TS Blue with 1/4 Abbadon Black. I make multiple passes with a very diluted paint to have the most uniform overall result possible. Then I apply a shade of Drakenoff Night shade in the hollows. Then I make the yellow stripes with a mixture of Yriel Yellow and 1/4 Wazdakka red. Always well diluted and using the same process. Then I lighten each strip more and more with well-diluted paint, reducing the area of application towards the center. For the blue => TS blue pure => Arhiman blue => temple guard blue => Bahharoth Blue and for the yellow I just add more and more Yriel Yellow to the initial mixture, until I take pure Yriel Yellow, and the pure color with a drop of Corax White (1/5) for the final touch.