It depends...
Firstly, I´m severely red/green colourblind which gives me the effect that on the scale from dark red, through orange to yellow, the closer to yellow, the better I see it.
This does not (I hope
) apply to rescue aircrews but as 12% of the worlds male population have it, it can be something to consider.
For me, dark red is virtually invisible!!
When hunting we often wear red garments. I tell my huntingbuddies that if you want me to see you, wear orange!
Why is huntinggear blaze orange and not yellow? Because in the spectrum that deer can see, red, orange and green blend but yellow and blue stand out. If you want to be seen by people but not animals, wear orange but not yellow.
I used to do mineclearing for the UN and the visibility of colourmarkings was considered important...
The only colour NOT to be found in nature is blue!
Blue is also the first colour to fade when daylight goes low...
(This could be a problem during mineclearing operations as a temporary safe lane in a minefield would be marked by spraypainting the outlines on the ground blue. I would walk out to the end of the lane and glass the area for telltale signs of mines. When it got to to dark I would put down my binos and turn around to walk back. The blue lines are now almost invisible...)