How to paint lava effects (like this one on eldar helmet :)
1. Base is Merchanite Red (GW)
2. First of all you have to sketch lava crackles using Blood Red (GW)
3. Next step is to improve crackles using watered down Blazing Orange (GW). Remember to paint with orange into crackles connections, and do not repaint previous red pattern completly.
4. In this step paint some glazes of Ember Orange (P3)
5. Almost finished, now it’s time to add more yellow color into pattern. I use Sulfuric Yellow (P3)
6. …finally add White spots randomly into center of intersections .
7. Now it’s time for shading what give more 3D look. It’s a last time you can correct any mistakes using base color. After shading it will be pretty hard. For shading I use Nightshade Purple (Reaper), Blood Red (GW) and Black mixed im proportion 20/60/20. Now add lot of water – it should be “colored water” 80% or more of water in this mix. Gently put some glazes using small brush and take care not to paint crackles. It need some patience but effect is worth it. Hope everything clear :)
Also you can check how to make lava base tutorial.