First of all, Kizaru uses lasers that explode, secondly, why exactly would light disappear in dark places? That's not how light works? How does light even exist in the universe if the universe is dark?
That works towards my point though? Kizaru's lasers burn through the target and explode, yet the light shines through Luffy's without harming them. The light coming out of Luffy's mouth is literally transparent, which is different from the compressed light forming an explosive laser, which is plain yellow.