Dammit, Legolas. We get it, I cut myself shaving, you don’t have to be all dramatic!
On a more serious note, it’s worth noting that there is a very noticeable difference in the sun when there’s is a fire nearby (likely a sign of battle as well).
Those who live near wildfire prone areas should be able to confirm.
For example, see this image that a user posted here:

Edit: It’s hard to capture it in a photo, but the sun itself becomes a very noticeable deeper shade of red when seen through smoke.
Plot twist: Legolas knows; but he also knows that blood is spilled every night in this God-forsaken world.
Well, you know what they say:
The blood of my enemies never sets on Middel Earth
Legolas is just tired of trying to explain why a red sun is different. Humans and dwarves just don’t have the photoreceptors to see green.

