In fire, light is latent and heat is dominant.
In light, heat is latent and light is dominant.
So in light, heat is yin and light is yang
whereas in fire, heat is yang and light is yin.
Taking everything into consideration light is yang and fire is yin
but where light meets resistance there is friction and friction creates heat and heat creates fire.
So in conflict fire is yang and light is yin.
But in reasoned conversation light is yang and fire yin.