Although it has a witness that it is a poisonous mixture of sulfuric gases and not lava which makes vivid colors. It is an incredible Indonesian volcano that it burns like a vivid bright blue color due to the sulfuric gas combustion. The Volcano That Burns Blue!
This great Big Story has explained how it is a wonderful combustion. From Indonesia to Java this Kawah Ijen is an incredible whole volcano that it is burning a beautiful bright blue light. What happens when the sulfur has cools down.
Source of image & video - https://laughingsquid.com/volcano-that-burns-bright-blue