Why were the headlights blue? Need some historian help!

I am reading a novel called The Madonnas of Leningrad so I can join a book club at our local library next week. It is not my typical read by any means, but so far I am enjoying it.

Anyways I came across this part of the book

dd

And so I need some help. I cannot find my answer online.

I am looking for more in depth answers or insights to the Russians using blue headlights to avoid the Germans in 1941 as the Germans approached Leningrad. I am just wanting to know more about this topic and other things that developed. I never thought of an invasion impacting the color of headlights.

Anyone have any guidance or insight?