Why are vent cowls / vents painted red on the inside?

[s/v Faith]

Ok, I have been on / around boats and ships all my life.

  I have noticed many private / pleasure boats have the cowl vents and air intakes painted red......

  Any one got the 'google fu' to find out why that is?

[CharlieJ]

very good question. In fact, Laura and I were asking ourselves the same thing just a week or so ago, when we were trying to decide to repaint the interior of the cowl vents we just acquired.

Can hardly wait to see an answer :D

[Captain Smollett]

Just a WAG, but could it be so they contrast with the surroundings to make them visible/noticable?  I trip over mine all the time, and they are NOT painted red on the inside.   ::)   ;D

[Lynx]

I have seen them green and red before. There may be a marine law about this somewhere/when.

[BobW]

Haven't found the answer to the question, but I did find this article on the basics of ventilation:

http://www.westlawn.edu/news/WestlawnMasthead01_April07.pdf

You need to scroll down to page 3 to get to the article.

[Parrothead]

Could not find a thing online. I will guess tho. Red lead paint.

[AdriftAtSea]

I would guess that the reason the interior of the cowl vents were traditionally painted red is that on older ships, they weren't necessarily mounted on dorade boxes, and any light that came up from below would ruin the deck watch's night vision, so the interiors of the vents were painted red to help prevent that.  That way, the light that did escape from the cowl vents served a useful purpose instead. 

I thought of this as I was entering my friend's darkroom, which has a black-painted hairpin turn corridor that acts as a light trap for it. 

[Godot]

I heard somewhere that barns are often painted red because red is the cheapest paint.  I don't know if it is true or not; but it sounds plausible.