Recipe from https://www.goonhammer.com/how-to-paint-everything-adeptus-custodes/

1. Basecoat the jewels with Thousand Sons Blue
2. Highlight almost the entire jewel with Ahriman Blue (leave some of the previous layer showing at the edges)
3. Highlight (“darklight?”) the top-left of the jewel with Incubi Darkness
4. Highlight the bottom-right of the jewel with Temple Guard Blue
5. Highlight the bottom-right of the jewel with a smaller layer of Baharroth Blue
6. Final highlight the bottom-right of the jewel with an even smaller layer of Baharroth Blue mixed with White Scar
7. Apply a dot of White Scar to the top-left of the jewel (the part painted with Incubi Darkness ). This should be a literal dot. Don’t worry if you mess up: Re-paint the area with Incubi Darkness and try again!